An archived album cannot be modified in any way. If you want to reactivate an album that you've archived, here's how.
To reactivate an archived album, locate it by going to the Albums page through the link in the top banner and then filter by archived albums.
Having opened up the archived album, click on the box icon in the panel to the left to unarchive it. You have to click twice to confirm the action. This will make the archive active again so that you can add and remove content to it.