C# 2010 web form app

I prefer by understanding module(s) and what kind of coding has been done with what tools/frameworks. Since you asked for web form it may have some scripting, third party API/tools etc. and they also require to understand. 

Finally you go inside coding/ flow by debug. Once you understand these things, it is very much easy to do maintenance.

For example someone has written some jQuery code to trigger some event on some controls and doing some kind of stuff using ajax calls, but and you are wonder and looking inside code behind to find that. But if you have gone through the kind of job is done before putting your head into code behind, you would be in better position to go at exact position for changes.