In your Image Manager, click on the name of the series or gallery on the left side menu.
Removing an Asset from a Gallery or Series
Select the asset or assets you want to remove, right-click, and select "Remove from gallery/series."
Renaming a Gallery or Series
At the top area (above the thumbnails), click on the name of the gallery or series to edit it.
Deleting a Gallery or Series
You can delete a gallery or series from the "Actions" menu in the top right corner.