This is an easy service you can do on your BMW telelever style bike. Depending on whether you have a GS or a GS Adventure the volume of fluid you'll need to replace will vary. Please do your due dilegence. The instructions here are for my bike which is a 2016 R1200GS (not adventure). It goes by the platform name of K50 in the BMW line.

I mostly used this procedure inspired by Capt Max ADV from this youtube video - Capt Max ADV BMW R1200GS Adventure Fork Seal Replacement

Rather than reinvent the wheel, you should just watch his video and I'll highlight what things I did differently to make this cleaner and easier and I'll provide a list of tools and parts you need to do the job.

Procedure

• Do one fork (stanchion) at a time! This is extremely important as this method is done without fully removing the wheel and fork and you need one fork leg to stabilize your front wheel.
• Be careful to avoid metal to metal contact! You should be able to remove the seals with the puller by putting the metal edge of the puller in the middle of the rubber seal and pry/pull it out to avoid damage to the fork. Scratching your fork could very easily require to you purchase a new fork (stanchion) as a leak will occur with fluid flowing out through the groove you created. Use a rag on the opposite of the fork where you're prying (applying leverage) from to avoid any damage.

Removal:

1. Remove plastic cap from top of fork - Pry with flathead screwdriver or plastic wedge
2. Use 14mm socket to loosen nut while using a 19mm wrench to prevent fork leg from spinning
3. Remove air bleed star nut
4. Gently guide stanchion down and move steering bar out of the way. Now you can pull the stanchion out. Once the stanchion is free of the fork leg, let the stanchion rest for a few seconds to let the oil flow into the fork leg.  Use a rag when removing to prevent drops of oil getting on your bike.
5. Rest fork leg in oil drain pan to let remaining oil drain.
6. Remove Dust Seal - The seal puller can help with this, but it pops out pretty easily
7. Remove Circlip using a Pick Tool
8. Remove Inner Oil Seal/bushing - This you need the seal puller - Place a rag on the edge where the puller rests against the fork and make sure the metal edge of the puller is pressed into the seal itself.  It's not necessary to get behind or underneath the seal and doing so will risk gouging the fork leg which would be really bad.
9. Remove the Washer
10. Use 500ml syringe to remove oil. Make note of amount removed.  It should be close to 485ml. When removing the hose have a rag in hand to run down the hose and then use it to cap the end of the hose to avoid getting oil everywhere.  Dispose of oil into your drain pan.

Reassembly:

1. For 2013-2017 GSW fill oil to be 133-137mm from top of fork leg. This should be 485ml provided you got all of the oil out with the syringe.
2. Apply small amount of light grease to washer and install
3. Apply small amount of light grease to outside, bottom and top of of inner fork seal and install. This is the most difficult part. You can use a bearing drift kit, or the old seal to help get even pressure and get the seal fully seated.
4. Install circlip. If the circlip isn't going in place, it's likely the seal installed in the previous step isn't fully inserted.
5. Apply small amount of light grease to Dust Seal on all but the top (exposed) surface and install.
6. install stanchion by sliding into fork. It should go in easily if you are sure to have it lined up and straight.
7. With stanchion fully inserted line up the steering bar so that the stanchion lines up and then pull the stanchion up and install the 14mm nut. Rotate the stanchion so that the bleeder valve faces out so that you can easily install next. Use the 19mm wrench to secure the stanchion from spinning and use a torque wrench to tighten 14mm nut to 40nm.
8. Install the air bleeder valve.
9. Use a marker to leave a mark on the 14mm nut and the inside of the assembly so that you can inspect later to be sure it hasn't loosened.
10. Replace plastic caps.

Now just repeat the above steps for the other fork leg and you're done!