Change Parent Of Child Record To New Parent

I want to go into a child record and change which parent it belongs to.
So my child table "Machine" has a field "CustomerID" linking to the parent table "Customer"
I'd like to add a new dropdown which lists all Customers and also a button "Change Customer"
The user selects the Customer from the dropdown and clicks "Change Customer" to update the Machine with the new CustomerID.
How do I

• Add a dropdown displaying Customer.name with index Customer.CustomerID
  
• Add the button event to capture the CustomerID selected
  
• Update the Machine.CustomerID with the new CustomerID
  

Thanks for any help.