Repair sealing inside toilet tank that leads to bowl
I have done this. The cylinder thing is called the canister. The red seal should stay attached to the part below, that the canister rests on. Mine was squishy and I was able to temporarily reshape it until I got a replacement.

The entire tower part rotates about 1/4 turn counterclockwise, then lifts off easily. It seems fragile, as if it might break, but mine didn't. There are probably instructions on Kohler's website. 

I know of no other option than to pay the overpriced $7 or so for an official replacement seal at HD or Lowes. The new ones are yellow and supposedly last longer.

Btw, if you ever replace the Fluidmaster fill valve that came with the Koehler, be careful that the new fill tube (white in the original) is not of a narrower OD, or else it will eventually come loose and spray sideways, which ends up on the floor somewhat. I fixed mine by wrapping with tape to make it have a greater diameter.