EDIT:
I've figure it out: How to close the POS sessions after some digging. Man, UI really made me feel like dumb dumb.

EDIT2: The tax line still display: "Tax 15%" dispite the product tax being 10.75%. Please fix.

I found the setting for taxes in Accounting app. However, entering an amount %-age and saving prompts a Invalid Operation: "It is forbidden to modify a tax used in POS order not posted. You must close the POS session before modifying the tax." I've closed all sessions and re-open Accounting to edit tax, but still the same Invalid Operation.