HI,
check what is the field selection assigned for you PO document type in SPRO-MM-PURCHASING-PURCHASE ORDER-DEFINE DOCUMENT TYPES . here you can check 'field selection' as in below screen
then goto SPRO-MM-PURCHASING-PURCHASE ORDER-Define screen layout at document level
and select the that "field selection" and then "terms of delivery and payment" here you can change accordingly as below screen.
