UPDATE 
tblRegDecreti 
SET 
tblRegDecreti.ProgARag = Nz(DMax("tblRegDecreti.ProgARag","tblRegDecreti","tblRegDecreti.AnnoIDProg=[Maschere]![frmPre-RegDecreti]![AnnoIDProg]"),0)+1
WHERE 
(
((tblRegDecreti.ProgARag) Is Null) 
AND 
((tblRegDecreti.IDDecreto)=[Maschere]![frmPre-RegDecreti]![IDDecreto])
)
;