Access the amplifier's control panel. While this may be different from amp to amp, most electronics chassis are fastened to the top panel of the cabinet by four screws. Frequently there is a cross-piece behind the chassis, protecting tubes and transformers, that needs to be removed as well.
Slide the chassis carefully out the back of the cabinet. In some cases, this may require the removal of knobs and jack hardware from the face of the amp, but in most cases, the face is part of the chassis assembly.
Locate the jack being replaced and unscrew the mounting hardware on the face. De-solder the connections at the jack itself, rather than where the wires connect to the internal electronics, taking note of how the wires connect to the jack.
Solder the wires to the replacement jack in the same orientation as the original. Replace the jack into the face and fasten it in place with the mounting hardware. Slide the chassis back into the cabinet and replace the mounting screws. Re-attach the cross-piece if this applies to you.