For ASP.NET projects, to be able to modify code behind files, you need the following:

1. Under Tools->Options->Debugging->Edit and continue make sure that Enable Edit and Continue option is checked.

2. Under  your project properties (right-click on you project in Solution explorer and select Properties...) on Web tab make sure Enable edit and Continue option is checked.

Then start your project and press pause or activate a breakpoint and you are able to modify code-behind files as well as .aspx files.