I'm interested by editing/erasing/copying/etc... code lines in *.prc file ( executable for Palm OS ). I tried to open prc files with text editors ( notepad and others ) but I cannot see code just symbols... So, I don't know... what I want to do is... possible? If it is, how can I do it? With an IDE? Without?
I would like to notify that I'm a beginner in programming, I looked for an answer to my question but I have not found, maybe because it's a too simple question. If it is, I'm very sorry!
Thank you!

There are no "code lines" in a PRC file. It is a binary executable, a product of compilation; you would need to either obtain a source code (which you could then edit and compile to make a new PRC), or you can use a debugger to inspect it as-is (which requires a very high level of skill).
See here for more details. In general, you'd need to learn C first. I would highly suggest you start learning programming on another target, since you will not find as abundant resources for PalmOS as you will for Windows, OSX, Linux, iOS or Android. When you are fluent in C, you can attempt to figure out how to use it for PalmOS.