Open your database in phpMyAdmin, locate the jos_vm_orders table and open the table for browsing, usually by clicking on the browse icon in the first column after the table name (jos_vm_orders).
Then open the last order in the list for editing by clicking on the pencil icon on that row. The top field shown in order_id and you need to edit number shown in the Value box to a number one below the value you want to see on your next invoice.
Then save this by hitting the Go button at the bottom of the page with save selected in the action option box (usually save is there by default).
Then place a new order and you should find that you now have a better looking order/invoice number.
The problem with precise instructions is that you may have a different version of phpMyAdmin on your server, but the principle is the same.
If you are not used to editing databases then a good idea is to save a backup copy first. Use the export option to do this.