Hi,

It is not possible to update the PO in interface. You need to use api to update

PO_CHANGE_API1_S.update_po 

Sample code to update

set serveroutput ON;
DECLARE
l_result NUMBER;
l_api_errors PO_API_ERRORS_REC_TYPE;
BEGIN
-- This needs to be changed according to your environment setup.
FND_GLOBAL.apps_initialize ( user_id => XXXX,
resp_id => XXXXX,
resp_appl_id => XXX );
mo_global.init('PO'); -- need for R12
l_result := PO_CHANGE_API1_S.update_po (
X_PO_NUMBER => &po_num,
X_RELEASE_NUMBER => Null,
X_REVISION_NUMBER => &rev_num,
X_LINE_NUMBER => &line_num,
X_SHIPMENT_NUMBER => &shipment_num,
NEW_QUANTITY => 25,
NEW_PRICE => Null,
NEW_PROMISED_DATE => Null,
NEW_NEED_BY_DATE => Null,
LAUNCH_APPROVALS_FLAG => 'N',
UPDATE_SOURCE => 'API',
VERSION => '1.0',
X_OVERRIDE_DATE => Null,
X_API_ERRORS => l_api_errors,
p_BUYER_NAME => Null,
p_secondary_quantity => Null,
p_preferred_grade => Null,
p_org_id => &org_id
);
dbms_output.put_line ('l_result' || l_result);
IF (l_result <> 1) THEN
-- Display the errors
FOR i IN 1..l_api_errors.message_text.COUNT LOOP
dbms_output.put_line ( l_api_errors.message_text(i) );
END LOOP;
END IF;
END;

commit;

Hope this helps