How do you repair a stihl chainsaw carburetor?

That depends on the model! Most, but not all, are made (engineered) for the mechanic's sake. There are exceptions like the 015, 020, MS200, etc.

If it's an 066 go 1 turn out from seat for high and low. No 064 to my knowledge.

remove the cover plate from the back of saw right above the throttle trigger remove air filter and you will see your carberator

Check out the related link below for information.

If everything else is clean and working, try turning gentle the HI LO screws clockwise till the end and then unscrew 1 and 1/8 back! That should do it!