This is not a standard procedure. Internal document numbers are determined by document series. You will encounter them in the Sales, Purchases, Money, and Accounting modules. If you need a different document series, do not modify an already registered and active document series — always create a new one instead.
If it becomes necessary to manually intervene in internal numbers, you need to enable this feature in the menu Company – Settings – tab Other, and check the option Allow manual overriding of internal document numbers.