You can edit your tags under Contacts->Manage Tags on the left menu.
You can not directly delete a tag, but you can use the Merge Tags to acheive this. Merge Tags will transfer the contacts from one tag, to the other tag, then delete the original tag. It's necessary to Merge instead of Deleting tags in order to make sure contacts are not lost from a tag, which could affect email newsletters you have scheduled.
Feel free to check this video for more information on the Tag Manager:
If you have a deleted signup box, you can access it from the signup box screen by clicking the white button that says "Deleted Sign-up boxes". You can access the signup box there and remove the tag. Then you will be able to delete the tag.