Don't delete invoices. That's not a joke. For auditing purposes, you don't want a "gap" in your invoice number series and the typical way to handle this is to "void" an invoice rather than delete it. You can open Manage | database | fields and check the field options for your invoice number field. Most likely it is an auto-entered serial number field and you can adjust the "next serial value" setting.
But note that any related records base on a relationship that matches by invoice number will no longer be linked to an invoice if you change the invoice number.