How to set Sales document prefix number

You do it on the form itself.  Click in the number block and format the number as you wish, then save it, that form will continue using that format from that point on.  Each form has to be done this way individually.

note that you must have customer invoice numbers set to on in company settings

thank you @Rustler 

Can you help me further by showing the screen caps / step by step?

I did it once in the beginning to set up and forgot where to reset the invoice numbering sequence with a new year.  Thank you so much.

Jacky

jacky it is a new year, live life on the wild side and just click in the invoice number block