Open Invoice form (create plus (+) sign at the top right > Customers > Invoice). Click on the little Gear (⚙) icon at the top right of the Invoice form. Select “Custom transaction numbers” box. Close the right panel by clicking “X” sign. It will produce a box to fill invoice number now. New transactions will auto-populate a new number in the sequence based on the last number entered. You can turn this off if there is no need to assign new number every time you create an invoice.
For more details regarding customizing the invoice number or other transactions check out this article.