You can rename an album by going to photos.dropbox.com.
To rename an album:
- After signing in to your account, go to photos.dropbox.com.
- Click the album you want rename.
- Click on the album name at the top of the page. It will change to an entry field where you can edit the text.
- When you're done editing, hit Return or Enter on your keyboard, or click anywhere outside the field.