Hi Rahul,
see SPRO: Materials Management -> Purchasing -> Purchase Order -> Define Screen Layout at Document Level, select field selection PT9F "Service item, PO", under group "Terms of delivery and payment" you can change the control for field label "Incoterms part 2 item".
Kind Regards
TomT
PS: Out of curiosity, what's the point of having Incoterms for Service item?