While there may be a better way to do it, you can directly update that tag in the db.
update cms_Tag set TagName = 'new name' where TagName = 'To Read' and TagGroupID = (whatever group that tag is associated with) .
This will update the tag name for all of the documents using it
Alternatively you can add the new tag to the pages that contain the "To Read" tag name, and remove the "To Read" tag from those page manually if there arent too many. To find all of the pages that contain the tag name go to Tag Groups > select the green pencil next to your tag group name > Tags > select the green eye next to the tag name "To Read" . This will list all of the pages that are using that tag. Click the green pencil to edit the page