1) In the Backstage View, select "Advanced Settings => Codes" ("Administrator >> Codes" in Projetex).
2) Click the "Next Codes" button and set your custom number. Please keep in mind that the number can not be lower than the latest number of an existing record of that type.

You have to set next codes for Quotes, Client Jobs, Invoices and Payments as well, if you want them to start from your custom number as well. You have to perform this for each client by clicking the Next Codes button on the Client Profile tab.