Turn off water to the toilet at the valve behind it near the floor. Twist the valve clockwise completely.
Flush the toilet several times to empty the tank.
Lift the cover off the tank and set it aside.
Disconnect the small chain that extends from the top of the flapper to the top of the toilet handle's linkage arm. Note that the chain is secured to the linkage arm with a cotter pin. Pull the cotter pin out of the linkage with pliers.
Remove the four screws located at the base of the flapper assembly, turning counterclockwise with a screwdriver. Lift the assembly out of the tank.
Position the new flapper assembly into the tank and install the assembly's four screws.
Position the flapper's chain against the toilet handle's linkage arm, then slide the cotter pin through the chain and the arm. Spread the two ends of the pin with pliers to secure the chain in place.
Lower the cover onto the toilet.
Rotate the water valve counterclockwise to turn on the water and allow the tank to fill with water.