Mattias Noback gives some good tips for refactoring legacy code.

"Combing" legacy code by untangling the strings is a good way to improve it and take back control over it. In existing legacy code, you should stop (re)using existing methods, making them ever more generic. Instead, you create new methods, and copy code from existing ones, allowing you to simplify this copied code and end up with a manageable class.

https://matthiasnoback.nl/2018/04/combing-legacy-code-string-by-string/