Here’s How to Flip between a Mac App’s Windows with a Keystroke
If you work like me, it's way too easy, in some apps, to end up with heaps of open windows. You can usually switch between them by clicking those you can see or by choosing one from the Window menu, but wouldn’t it be nice to cycle through them from the keyboard, much as you can cycle through active apps with Command-Tab? You can! By default, the shortcut is ...
Command-Backtick (the key above Tab)
Additionally, if you want to change it to something else, open System Preferences > Keyboard > Shortcuts > Keyboard and look for “Move focus to next window” in the list. To change the shortcut, double-click right on it in the list and type your preferred alternative.