Note: this is for Simple Invoices 2010.2 and above

Say you want the have the invoice number start at 1000 instead of 1

Open phpMyAdmin, go to your Simple Invoices database, then find the si_index table. Then click browse, the data should look something like

edit this row and change it to below

or just execute the below SQL

DELETE FROM `si_index`;
INSERT INTO `si_index` VALUES('999','invoice','1',' ','1');

refer:

• http://simpleinvoices.org/forum/discussion/1287/change-invoice-number-on-beta6/