If you have already been using numbered forms and wish to continue in the same sequence, double-click on the "No:" label (just left of the blue number field). Doing this will temporarily unlock the blue field so that you can type in a starting number other than "000001". Make sure you type in a six-place number including leading zeros.
Save the document, and confirm that it shows up in the saved documents list at the bottom of the screen.
From here on out your document numbers will go up from the starting number, in line with your old system.