I'm assuming you have a problem with water staying in the bowl? If that's the case the first thing to do is turn off the toilet at the breaker and turn off the water. All the following is done with no vacuum at the bowl and the toilet full of water. Then put some dawn liquid soap into the bowl and as you step on the handle brush the inside of the gasket, make sure it's a round toilet brush. After you've cleaned the toilet, turn on the toilet and water. Let the toilet fill and flush. Wait to see if this solves the problem.
If not, turn off the toilet and water, flush the toilet and release the vacuum.
Replace the gasket by removing the toilet cover, then the toilet from the flange. Remove both gaskets, ball valve (1) screw and ball valve cam it's attached to. The brass cam is also attached to the foot pedal. It has two "O" rings and either the cam needs to be replaced or the "O" rings (2). Use silicone grease on the cam and a very thin coat n the gaskets and ball. Don't over tighten the toilet to the flange, it should be very snug but remember the flange is plastic (ABS) . Also, you are not removing the flange from the base unless you suspect it's got a crack in it.
If you are thinking of just replacing the ball valve,, don't, replace everything. The ball valve, both gaskets, brass cam. I hope it's just a cleaning, but if not, it's very simple and takes about one hour.
Regards,
Bow Tie