i tried bypassed adding a product model but it isnot work, do you have a code example for that?

Yeah, that’s a common one — Odoo blocks the UoM change if there are stock moves linked to the product. Easiest workaround is not to touch the main UoM but use a separate field or handle the conversion in POS directly. Otherwise, you’ll need to bypass that constraint in code, but do it carefully to avoid messing up inventory consistency.