changing invoice numbers

-- url of the page with the problem -- : www.comjoo.com
-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.4.8

Dear Hikashop-Team,

I would like to change the invoice numbers - at the moment it consists just out of numbers (chronological). I would like to have at least 3 letters and 3 numbers (a bit like the order number but with 6 signs). It shall not be chronological - if it is not possible with letters, the invoice numbers shall at least consists out of 8 numbers. Can you tell me where exactly I can change it and how I can change it?

Thank you!