Hi @kristinubute,
If you want to leave the existing invoice numbers as they are, and simply want to set your next invoice number to 6000, you can go to WooCommerce > PDF Invoices > Documents > Invoice > Next invoice number.
If you want to renumber your existing invoices, starting from 6000, we offer a small tool that can help you achieve this. You can download it here: WooCommerce PDF Invoices & Packing Slips Number Tools
After installing and activating the add-on you will have an extra tab called ‘Number Tools’ in your PDF invoices settings (WooCommerce > PDF Invoices > Number Tools). If you click on ‘Tools’ you will see the option to renumber existing invoices within a certain timeframe. Before renumbering you will first have to set the next invoice number to 6000. Then set your timeframe and click on ‘Renumber invoices’.
You can set the invoice number to be equal to the order number via WooCommerce > PDF Invoices > Documents > Invoice > Display invoice number. But this means you will not be able to set it to start at 6000 and that gaps can occur in your invoice number sequence, which in most countries is illegal.
To understand the difference between order numbers and invoice numbers, and why we would always suggest to use our separate invoice number sequence, please read the following article: Invoice numbers explained