How do I edit a file?

Use a text editor. Perhaps the two most popular text editors are vi (or its improved form, vim and emacs.

Emacs has a good tutorial built into it. Start emacs, and type C-h t. (This means press ctrl-H, then press t and sit back.)

vi also has some online help available. Start vi and type :help followed by the enter key. A quick web search produced this vi tutorial. Here is another.

Which you use depends on you. You will have to try both to find out. The chances are that you with love one and hate the other. Text editors can be a very personal thing, and there have been many flame wars started over stupid things like text editors!