Complex user story that goes beyond a week

Hi,
Our team is currently working on a particular user story which involves modifying an existing feature of a huge existing program (more than 30,000 lines of codes).
The existing code was written by a programmer in the past and the feature involves codes in multiple locations in the program.
So for our team to modify that, we need to understand the ins and outs of the program to ensure we are modifying the right places and not breaking anything which is not supposed to be touched.