Once a Purchase Order (PO) has been receipted, the allocated warehouse the the PO has been receipted to cannot be edited or changed. 

If the PO has been receipted to the wrong warehouse, you can correct this by either:

• Creating a Warehouse Transfer to move the stock to the correct warehouse, see Add Warehouse Transfer.
• Complete the PO, create a Supplier Return to reverse the PO and remove the receipted stock, then create a new PO and receipt it to the appropriate warehouse, see Add Supplier Return.