What is needed to fix or replace a cracked old seal on a refrigerator door?
The refrigerator gasket is replaceable, but you'll need to find an exact replacement, possibly from the manufacturer.

Before doing that, make sure that the door hasn't warped, which would cause one of the corners to not close tightly. To straighten a warped door, loosen the screws that are beneath the current gasket (when you open the door, you can pull the gasket away to expose the screws). With the screws loosened, you can pull or push on the corners to get the door back to straight, and then re-tighten the screws.

If you decide to replace the gasket, then the process is to remove the existing gasket by loosening those same screws, and then installing the new gasket, starting at the top and working your way down and around the door. Designs may vary, but it should slide behind a metal or plastic clip/gap that is tightened via the screws. When you're done, verify the gasket is straight and tighten the screws in each corner. Check that the door isn't warped, and correct it now while it's still easy to fix, and then tighten the remaining screws.

When finished, double check for any gaps. I've seen suggestions to use a flashlight inside the refrigerator, close the door, and see if the light can be seen with the room dark. I've also seen suggestions from talcum power to a thin coat of petroleum jelly to lubricate the seal and/or prevent sticking. Follow your manufacturers suggestions or use your own best judgment there.