[xupybd]

I’d say the most important thing is to learn the domain and the business you are working with. Never assume the code is doing things the right way for the business. Get to know your client really well and try to understand what they need to software to do. Keep them in the loop as much as possible.

[fendy3002]

This should be the best course of action before diving into technical aspect. Look for any updated business documentations. If there aren't any, ask the users of systems to find use cases and replicate them in dev system. Try to document the business flow and emulate any hidden behaviors. Last, open the codebase and matching business process with code, and comment everything possible.

Then you can begin fixing or refactoring with use cases / test cases in hand. If time isn't possible for that, try to shift the responsibility to the one giving you task (pm).