Hey, my friend told me something that he knows how to hotrod an amp to give it more gain. Is there instructions on how to do this?
yeah. They call it an SD-1 upgrade.

Take it to a tech man. A JCM900 is too nice to do on your own if you have to read instructions on how to do something like that ya know.
Hot Rodding doesn't always mean more gain, sometimes it's a change to the tone stack to clean up the sound. Sometimes it's adding adjustable bias. Basically it's something that maybe should have been done before to make the amp better sounding or more user friendly. There's a whole industry based on hot rodded amps, see Splawn Quick Rods (hot rodded JCM800).

You can change tubes, resistors, capacitors but if you don't know what you're doing you could break your amp or hurt yourself.
It's a fine line between clever and stupid.