To replace the flush valve seal on a Kohler toilet, start by using turning the water shut-off valve to the right to cut the supply to the toilet. Remove the lid from the top of the toilet tank, and drain the tank fully by holding the down the handle of the toilet until the tank is completely empty.
Next, remove the chain and lever connecting the flush valve to the handle in the inside of the tank. Pull out the refill tube from the top of the flush valve canister. Hold onto the stem inside of the canister and twist it counter clockwise until you can remove it from the canister, and then remove the rest of the canister from the toilet.
Along the bottom of the inside of the toilet tank there is the round rubber flush valve seal. Remove the old seal, and put the new, matching seal in its place. Put the canister back, positioning it on top of the seal. Insert the stem into the canister, and twist it clockwise to lock the canister into place. Insert the refill tube into the top of the flush valve canister. Finally, reattach the chain and lever to the part of the toilet handle inside of the tank.
Turn the water supply to the toilet back on by turning the shut-off valve to the left. Make sure the tank fills completely, check for any leaks, and flush the toilet to make sure it functions properly.