It is essential to enable the options given below:
Company Setup> General Tab> “HSN Editable Page List” and select the “PO Approval Update" in the dropdown. Save it.
When the PO approver will go to the “POs for the Approver” screen. Then click on edit in the “Actions” field against the PO No. and edit the wrong HSN code to the correct HSN code. Now, click on “Update and Approve PO”.