If you need to edit the Category of an online gallery, click on the Collection you’re working on, and then click on "Galleries" at the top.
Next, click “Edit” to see your gallery's settings.
Navigate to the "General" tab, and make sure you have "Display on Portfolio" toggled to on.
This should then activate the drop down menu right below it, where you can select a Category that the gallery will be assigned to! 👊