Hello,
1 - If you open the official "after_end" view of HikaShop 3.2, you will see few lines of code which are dealing with the order ID.
2 - backend / order / changestatus
But please note that specific interface could evolve in the future since we more recommend to change the order status via the order details and not the order listing (specially if you are using some add-ons because you will got unwanted side effects).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.