Eh. Don't upload a whole new gallery if you just want to update a gallery. Now it's too late, but for the next time:
If you want to update a gallery:
Go to My Uploads and either click on the gallery name on the left or on "Manage" on the right of the gallery you want to update.
Scroll a bit down and click on Create New Version.
After that you can change the Main Gallery Title if necessary. (for example: removing the [Ongoing] indicator if you are going to publish the full version of a work) And you can update the Uploader Comment.
Then you can rearrange the order of images and also add and remove images.
If everything is done and you are satisfied, click on Publish Gallery. The old version of the gallery is still available to see as parent gallery. All tags which were set before are transfered to the new child gallery.