How do I apply a patch file? I downloaded a patch file from mailhandler module and need to apply the changes in order to fix the problem of pop3 accounts not deleting emails after download (import).  How do I install it?
Here are a few links that may help you learn more about patching:


Applying a Patch Manually
Making a Drupal patch with Git (also covers applying)
HowTo: How to apply a patch to a contributed module - Beginner's version (Windows)
Apply Patches on Windows (more methods)
Apply patches on Mac OS
How to apply patches for Drupal modules and files