Stored Procedure Group Invoice Records And Update Accordingly

I am a Newbie using SQL server and vb.net in VS2010. I am re-writing an existing application which is written in an old programming language. I have a table with invoice records waiting to be assigned an invoice number and invoice date. I have two ways of updating these records , the first way is the user manually selects which records/rows to be updated for a specific customer account number. This all works fine.

I am trying to write a stored proc which goes through all un-invoiced records (i.e., invoice number = blank) , it is to group records/rows by account number and assign next invoice number (which I already have astroed proc to fetch) and then when the account number changes to move onto the next batch of records for the next account.