Any decent text editor can be used to change the script files(.gd) as they are plain text. The community often uses either Sublime Text 3 or Notepad++. Generally you just need something that provides line numbers, since that is the simplest way to find stuff. Otherwise, function names are used for which the definitions can generally be found by searching for "func " followed by the function name. Syntax coloring for languages like Python and formatting for JSON files can be nice to have.
Edit: also make backup copies before you start, small mistakes can cause big problems.