If user selected lowest UOM (i.e. PIECE) while adding a new SKU, by default UM value will be 1. Which cannot modify. But if user wants to own value in UM, in that case they can create their own UOM and select during SKU creation and assign value to it.
But we highly recommend if it is PIECE, then UM value should be 1 always.
We hope the details above were helpful for you. Please feel free to share with us if require further assistance and will be happy to take it up.