How do I restore a previous script version?

How do I restore a previous script version?

For some reason I can't update my script all of the sudden. It was working fine a few days ago but when I tried to edit the script today I keep getting a null message. I can't figure out why so I'm wanting to restore the script to a previous version that worked just a few days ago. I can't find how to do that anywhere in Zoho's help. Can someone tell me how to restore a previous version?