Here's how to change the name, guest permissions and button order for sub galleries. You can also delete a sub gallery from here.
Open the Main Gallery where the sub galleries are - skip this if you're already in the main gallery. Example of a main gallery highlighted below:
Click the Menu button at the top right (purple circle with 3 white lines) to open the "Gallery Menu"
Click "Sub Galleries" on the menu list. You might need to scroll down depending on your screen size.
To change the name/button title or guest permissions for a sub gallery (or delete one), simply click the corresponding button (with the pencil icon), and make the necessary changes.
If you need to reorder the sub gallery buttons, click on the "Reorder Galleries" button