Turn off the power to your amp and unplug it.
Unscrew the back cover of your amp. Depending on the amp model, you may have to use an Allen wrench instead of a screwdriver. If you can't access the speaker from the back, unscrew or unbolt the grill of your amp.
Cut the two wires connected to the speaker. If you want to avoid cutting the wires and wish to preserve wire length, you can de-solder the wires from the original speaker.
Unscrew the speaker from its frame and remove it from the amp cabinet.
Turn on a fan and open any windows in your workspace. Solder fumes can be hazardous to your health.
Turn on your soldering iron. It will take a few minutes to heat up.
Apply a small amount of solder to the tip of your iron. This is called "tinning" the iron and helps it conduct heat.
Connect one speaker wire from the amp to one terminal of your new speaker. Do this for both wires and terminals.
Touch the soldering iron to the connection of the wire and terminal.
Apply solder to the connection between the wire and terminal. Do not apply solder to the iron itself. The connection should be hot enough to melt the solder onto the connection.
Fasten the new speaker to the cabinet, and reattach any framing elements you removed.