Firstly, make sure users have the following permissions:
- Go to Setup > Enter Profiles or Permission Sets in the Quick Find Box.
- Click on the name of the profile or permission set that you want to edit.
- Go to tab settings > Go to Custom Tab Settings > Update to 'Default On' > Save

P.S. Discover Auto Number Helper, the simple way to update Auto Number values in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N30000000puTcEAI