Unplug any cords connected to your amp, especially its power supply.
Unscrew the four screws holding your amps back wall in place. Peavey uses both Phillips-head and flat-head screws on its amps, so use the screwdriver that is appropriate for your particular amp. You will know which to use by looking at the screw heads.
Set aside the screws you've removed for use later.
Unscrew the screws holding the amplifier cone in place. This detaches the cone from the inside of your amp, leaving two wires connecting it to the speaker cabinet. Set aside the screws in a separate location than the ones you detached earlier so as not to confuse them.
Follow the cord leading from the broken cone to where it is attached to the amp's internals.
Detach the wires from the inside of the amp by firmly grasping where they are connected and wiggling them back and forth until they disconnect.
Attach the wires of your new speaker cone to the location where you just removed the old cone's wires.
Place the cone in the cabinet where the broken cone had been.
Screw the new cone in place using the screws you had set aside.
Replace the back wall of the amp.
Screw in the screws that had initially fixed the back wall of your amp.