Simple; if the jack socket has a whole at the back, unplug the amp, remove the chassis and push it out from the back of the socket. If the rear of the socket is filled in then you will need to replace the whole socket and wire in the new one.
They are not expensive and can be obtained from any component supplier. Best if you can get one of the exact type, but any type can be modified and wired in.
Hope that helps.
Guitarman
August 2008