if you are talking about the wax ring you remove the floor mounting bolts and replace the wax ring under it. first you want to shut water off then flush to empty tank and with a wet dry vac remove any water left . now you can remove it without any mess.
https://www.youtube.com/watch?v=FlJ3UQlkEdU
First you will need to obtain the kit or parts necessary for replacement, compare your parts to your existing before doing anything, you will need your model number and if you had a picture more help could be offered. Begin by turning off the water supply to the toilet, keep a small bucket handy to catch the water, disconnect the supply line at the toilet if your are doing a complete replacement. If its only the handle again a handle kit will need to be obtained for your specific model no need to turn off water but keep in mind your exact handle or button placement. When you have the correct parts its just a matter of remove and replace and take photos of before so you can reference what goes where if you get sidetracked.
if you flush and the water continues to run into the bowl the it is the flush washer that needs repalcing
If that water shuts off and the cistern fills and then continues so that it flows down the over flow pipe into the bowl the it is the water inlet control washer that needs replacing
