Thank you both for your replies, I have looked at the GEO Jobe Admin tools and unfortunately it just allows you to remove or add tags not update ones so I need to get a register of where all the bad tags are so I can update those ones.
I have found a way to export a list of all items in my organisation using the Geo Jobe tools and then mess about with it in excel but it is extremely clunky. Hopefully future releases for AGOL will include an option as an administrator to review and update all tags used by their users.