How to edit class files?

The answer to your question - how to edit class files is that it can be done in several ways -

Decompile .class to .java source (not necessarily original source, but equivalent), make your changes, compile again. There are many Java decompilers out there, I won't list them all.

Disassemble .class to "assembly" (usually Jasmin syntax), modify it, assemble back. Krakatau is a good tool that should be able to do it.

Use a tool that can modify/patch the bytecode directly in the .class file. I haven't tried it, but Recaf claims to be able to do it.

Hook the Java runtime interpreter to catch the moment it starts executing the target function and make it execute something else instead. This is somewhat implementation-specific but here's one example I know about which uses Java debugging protocol (JWDP): https://github.com/CrowdStrike/pyspresso