You can replace a flush valve by yourself, but it's much easier if you have help. Removing the tank bolts is quite a stretch for arms that are attached to the same torso, and lifting the tank off the bowl can quickly remind you of just how old you back is. Both these jobs only take a couple of minutes with someone else.
1. Loosen each bolt with a socket wrench on the bottom of the tank to turn the nut and a flat-blade screwdriver inside the tank to back hold the slotted screw. Corrosion can make separating the bolts and nuts almost impossible.
2. If the tank bolts won't loosen with a wrench and screwdriver, then cut them with a hacksaw blade. The bolts are usually brass, which is much softer tan steel, so this job isn't too difficult. Wrap the end of the blade with tape to protect you hand.
3. To remove the old flush valve, first remove the old spud washer from the bottom of the tank(insert). Then loosen the spud nut that holds the valve with large groove-joint piers. Turn the nut in a counterclockwise direction.
4. Clean any dirt or old pipe joint compound from the flush valve opening. Then install the new valve through the tank hole. Position the overflow tube on the new valve so that it will rest next to the fill valve on the fill valve.
5. Thread the new spud nut onto the bottom of the new flush valve, and make sure it sits flush on the bottom of the tank. Finger-tighten the nut, then securely tighten it with groove-joint pliers. When the nut squeaks against the tank it's tight enough.
6. Slide the new rubber spud nut, and push it down so it's flush against the bottom of the tank. Then spread a uniform (and generous) layer of pipe joint compound over the top, but not the sides, of the washer.
7. Slide rubber washers over the ends of the tank bolts, and coat the underside of these washers with pipe joint compound. Insert the bolts in the holes at the bottom of the tank and through the holes in the toilet bowl. Tighten the bolts.