An archived album cannot be modified in any way. If you want to reactivate an album 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 filtering by archived albums.

After opening the archived album, click on the box icon in the panel to the left to unarchive it. You must click twice to confirm the action. This will make the archive active again so you can add and remove content.