You need to follow the steps mentioned in the configuration guide - https://docs.google.com/document/d/1ePiIm0jtpzhmSoXad7q9H304uRchY_Sb4Kkh7WauohI/edit?usp=sharing
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