To me, there is nothing unclear about saying
"I would like to change my order from A to B." if that is what you want to do.
Whether that is the best thing to say would depend on the agreement you have with the manufacturer. Talking about cancelling and replacing, though not wrong from a language point of view, should not be considered the default way to amend an order. In many situations, cancelling an order might not be allowed or might incur penalties.