Fix Kohler toilet canister flush valve gasket seal

The Kohler canister-type flush valve gasket seal is replaced like this:

1. Flush the toilet and turn off the water supply.
2. Push down and turn the guide post at the center of the canister 1/4 turn in counter-clockwise direction to separate the canister.
3. Remove it from the tank.
4. Pull the gasket from the bottom of the canister and replace it with a new one.
5. Reinstall the canister by inserting the guide post, and lock it with ¼ turn in clockwise direction.