Need to alter the number? Thats easy.
You will first need to enable this on your account, click on Settings which is the cog icon at the top right-hand corner of the page.
Select Document & Logo Settings.
Select Numbering Rules on the far right-hand side of the page. Then scroll to the bottom of the page. You then need to switch on the Allow to overwrite document's Document Number button. Once you have switched it on, press Save.
You will see you can also add a different Prefix and Suffix to your document numbers.
Now open up the quote/invoice you wish to change and click on Edit at the top right-hand corner of the page, under the Options button.
At the bottom of the page, you will see Document Number. You will now be able to change the document number.
Your next quote/invoice number will be generated sequentially from this number.