[edimaudo]

Definitely a quagmire but there are some things you can do. Some helpful tips

Not every code is pretty so it should make for an interesting learning curve.

Walk through every line of code and create your own documentation using something like this - https://www.writethedocs.org/guide/writing/beginners-guide-t...

Break down the problem into sub-problems. YOU CAN use user stories if possible to understand the business logic.

Talk to your manager and set expectations. It seems they have given you a task that might be more than your currently knowledge set. It could a learning opportunity or a roaring disaster. Either way make sure you tell them what you can do right now and what may take you longer to do.