How to Switch Between Windows of Same App

This is done with alt-tilde.

You can change the shortcut from:

Preferences > Keyboard > Shortcuts > Keyboard > Move Focus to Next Window 

You can see the option highlighted in the screenshot.

You can change it to your liking. People generally tend to use the key right above Tab, which is generally ~. (could be a different key depending on your keyboard layout).

Happy hacking!