What kind of issue are you wishing to address? It doesn't cost anything to take them apart and clean all the contacts. If it is a functional deficiency, be aware that there are also relays involved with most components controlled by the handlebar switches. Euro Moto Electric  (EME)  has replacement switches on their website. 

https://www.euromotoelectrics.com/category-s/3406.htm

Other parts suppliers are available as well. Max BMW and Rubber Chicken Racing Garage come to mind. 

I have to ask as James has, what is the issue you are having?  

As for repairing these switches, they come apart very easy but don't go back together so easy. The issue is the tiny spring inside along with a tiny ball bearing that will, if not careful, launch themselves across the room or into the unknown.   I strongly suggest if you do decide to try to take things apart do so in a clear plastic bag so if they do take off they don't go far.   Putting them back together may need a third hand to hold things in place.  LOL at the factory they were assembled on a jig with proper tools we don't have.

Before I would take them apart, I would shoot some contact cleaner into them and work them a bit.  Good luck St.

I should add, I am certain about the spring and ball bearing in the left hand unit with the headlight off, park and on lever.  US bikes after a certain year got rid of this for the headlight always on requirement.  St.

Before taking the switches apart sweep the floor very clean and clean all unnecessary items off the bench.  Makes it much easier to find the spring and ball when they go flying.  I learned this lesson the hard way.  Other than the loss of parts rebuilding the switches is not too difficult of task.  You will find the factory grease in the switches has turned into a solid.

The contacts in the handlebar switches are very high quality and self-wiping (in effect self-cleaning). I think you will find either after great struggle or great expense, that the electrical fault you would like to cure lies somewhere else in the circuit.

I have found that connector terminal corrosion from 40 years of bike washing and riding in the rain has a much higher probability of being your issue. What I would suggest is that BEFORE you spend big bucks on switches or big bucks on headache remedies (because you took the switches apart) is that you do this...

Buy the smallest container of electrical connector anti-oxidation compound, such as No-Ox-Id by Sanchem, that you can find. But even the 1oz tub of this stuff is under $7 on Amazon. LINK

Open up your headlamp bucket, and with the aid of a bright lamp and a skinny pair of pliers (such as "duck bill" pliers), in an orderly fashion... disconnect every wire one at a time from the multi-color circuit board in the back of the headlamp shell. Apply a very small amount of anti-oxidation compound (the volume of one drop) inside the female terminal, and then re-plug the wire to the same prong from which it was removed.

This may cost you 20 minutes and $7, but the changes are usually miraculous. Horns get louder, lights get brighter... in summation, all sorts of good things start to happen.

Then do the same for the 2 fuse ends, all relay terminals, battery connections... in short... ANY electrical connection you happen to work on can be improved by adding a small amount of this compound during re-assembly. 

Hope this helps.

I asked what issue you are having that makes you ask about replace or rebuild?   Frankly, I would try cleaning the switch with contact cleaner first.   Yeah, switches do finally go bad, in mine the horn button quit and no amount of cleaning or fussing with it would fix it, it had to be rebuilt or replaced.   At the point of taking it apart, that turned out to be a useless endeavor.  The switch was just not going to work.  I suppose I could have soldered in new contact strips or "rebuilt" it more but I don't have the patience nor the soldering skills.  First time reassembly was difficult enough to make me realize I didn't want to take it apart again to do more "rebuilding" and try to put it back together again.

Some people have a lot more patience and are very good at taking things like this apart and putting them back together again.  If you are not one of these people, and you can't get the switch working by cleaning it, it might be best to source a replacemnet.

Richard says pull all your contacts and coat them, that is fine as long as you can keep things simple and work one wire at a time and your headlight shell interior doesn't look like a bundle of jumping snakes.   If you get a contact out of place inside there. you could have a fine time trying to see and fix it.    I personally don't mess with the wiring in the headlight shell anymore than I have to. I have never seen any corrosion inside a properly assembled headlight shell that would require pulling wires off to coat.  BMW didn't do it at the factory.   Anyway, even doing that might not fix your switch problem, just give you more to do. St.

I recently had headlight/turn signal problems. Previous owner described a bad switch. I ended up replacing the relays and it fixed the issue.

Somewhere there is a list of the order of operations to take when dealing with electrical issues, but I can't remember where I saw that. Disassembling the switch is not the first thing to do, rest assured.

Of course, being young and dumb, I did end up disassembling the switch a few times just to clean stuff and try and identify obvious issues. It's not hard to do on my 84' R100 but that spring and ball are definitely hard to get back in appropriately. Also, if you don't have great soldering skills, disassembly won't get you very far because there's not much to do after it's all apart other than clean contacts and re-solder the connections (which realistically look ok).

Best of luck

In my experience with my bikes, I have found relays quit more than the switches.   I have replaced three relays in 40 years and zero switches due to break down. St.