I understand you're looking to update the author for documents created under another user that has left the company. Unfortunately, this isn't supported out of the box and will require customizations to make it feasible. One potential option may be the Author Change Add-On FAQ. If you're interested in this or other customizations, I would encourage speaking with your Jive Account Manager to start a discussion with our Professional Services team.
I hope this helps. Let me know if you had any other questions on this.