Switch the amp off and disconnect it from any power as attempting to take it apart with power going through it can cause dangerous shocks. Look for screws at the back of your amp and remove them first. Remove the back panel. Pull it out with minimum force once the screws are removed.
Unscrew and remove the amplifier's reverb tank. Gently pull any wiring out without straining or breaking any as this will damage your amp further and potentially lead to a live wire inside your amplifier.
Unscrew the speaker from the back of your amplifier slowly. Remove it slowly once you have done so, being sure not to drop it as they are usually hefty but fragile. If dropped, any number of fittings inside could come loose, potentially permanently damaging it. Place the speaker somewhere secure until you are ready to screw it back in again. Unscrew the wood that the grille is attached to. This may require a slightly more fervent strength. Pull the slabs of wood apart and simply pull the grill out.