Replacing the canister flapper/seal
The canister flapper/seal is easily replaced.
First, turn the water supply off.
Observe the white hose that goes into the top of the guide in the middle of the canister.
Grab the guide and rotate it counter-clockwise one quarter of a turn.
Now you can lift the canister out and observe the red seal on the bottom.
You can now pull the red seal off the bottom of the canister and replace it.
Reinstall the canister by lining up the tab on the bottom of the guide with the slot of the flush valve and rotate it clockwise to lock it back into place.