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 Settings.
Select 'numbering rules' on the far right hand side of the page. You then need to switch on the 'allow to overwrite documents Document Number on Edit document screen' button. Switch this on and press Save.
You will see you can also add a 'Prefix and Suffix' to your document numbers too.
If you now open up the invoice you wish to change and click on 'Advanced edit' at the top right hand corner of the page.
At the bottom of the page you will see 'Document Number'. You will now be able to change the document number.