How would you maintain legacy applications
You need the book Working Effectively with Legacy Code by Michael C. Feathers.
Write tests as soon as you can. Preferably against the requirements (if they exist). Start with functional tests. Refactor in small chunks. Anytime you touch code, leave it cleaner and better than when you started.
Two things.
- Write unit tests as you have the chance.
- Once you have enough unit tests to be confident, start refactoring.
The rate at which you accomplish this may be slow.... Typically, you're supposed to "just maintain it" not fix it.
During the "learning how to maintain it" phase, however, you can write a lot of unit tests.
Then, as bugs are found and enhancements requested, you can add yet more tests.
It's Agile, applied to legacy.