Once uploaded, Galleries can be edited and images can be added or removed. Click on 'More' in the main navigation and click 'Galleries'.
You'll see a view presenting all of your galleries - click 'Edit' on the Gallery you'l like to edit.
Click the small trash button on individual images you'd like to remove or scroll below the image section to add more files.
Once you've completed your changes, click 'Publish now' to make them live or click 'Save draft' to continue editing later.