How do you replace the thermostat on a 1991 Mercedes 300SL?

Hello scrammy25,
I first, would like to apologize for the delay in my reponse to your question. Second, I have never replaced the thermostat on your make and model, however it's very simple.
1. Locate the top radiator hose2. Follow it to the block - you should a (bubble/round shaped object (thermostat housing) at the block side of the top radiator hose). This is where your thermostat should be located.3. Most thermostat housings have two to three bolts that secure it. Before removing it, it's a good idea to locate the petcock on bottom of the radiator and drain some (or all if you are replacing the fluid as well {good idea} ) of the fluid. 4. Remove the bolts (if readily accessible) and lightly twist and pull up on the housing. Sometimes it's necessary to use a small screwdriver to lightly pry up the housing as it may be "stuck" to the block - but be very careful doing this as not to "gouge" the surface of the housing or block, as this can create a potential leak. 5. Once this is removed, clean both the surface of the housing and the block, making sure the block is as dry as possible (some water will sit, and lightly spill over, this is okay). 6. With the housing surface clean, dry and free of all remains of the old gasket (use a straight blade from a box knife or such for lightly scrapping excess gasket), apply a thin coat of gasket sealer to the surface - place the gasket on, let dry for about 10-15 minutes. While waiting, apply a small coat of gasket sealer to the block side (where thermostat sits) and let dry as well 10-15 minutes. Remember, make sure not to use too much, as this can create "raised" areas, which will create a leak. 7. Once both surfaces have dried, insert the thermostat in it's correct placement - (spring side down, point (top looking side) up). Gently place the housing over the thermostat, and insert both bolts, tightening my hand first, then snug them with your wrench or ratchet. NOTE: Do not "over, or under-tighten these" as either can cause a leak as well - as soon you feel "tension" on the bolts, give them a quarter turn, then stop.
Once you have done this, let set for another 10-15 minutes, while you refill the radiator (if drained). Start engine (hose is on and secure to housing, if removed or loosened) and let engine reach operating temperature - watch the level of fluid in radiator (cap off, this is okay) and add as necessary. If you released any fluid, and added while engine was off, when starting the engine, the fluid level may drop once the thermostat is open at operating temperature, and if so, add water (or antifreeze) as necessary to fill (with engine running). Once the level remains steady, put cap back on radiator, and let engine run inspecting for leaks in all areas.
Please let me know if you have further questions,
carsandcomps

Find the thermostat (it is in the coolant line and will be close to the engine block) unbolt it and refit the new one. When refitted, run the engine to check for leaks and to clear any air locks.