There is not a way to mass delete tags. The tag must be deleted from all assigned clients in order to delete the tag from your tag list.

To delete a tag for ALL clients, click on Manage and then search for that tag:

Now click on each client, click edit and then click on the tag you want to delete.

To EDIT tags, you will have to delete the incorrect tag using the previous steps. Then create a new tag. 

To create a new tag for multiple clients, check the box to select all the clients you want to create the tag for.

Click on the pad and pen icon at the top of the same column to edit.

Then type in the new tag or choose from the list of tags that come up.