Ok, hab's rausgefunden, so geht's:

update arpfda y
set ypakkz = (select a#akkz from artbez where
a#fa=999 and a#nl=999 and a#arnr = y.yparnr)
where ypfa=999 and ypnl=999 and
yparnr > 839999999 and yparnr < 841000000 and
yparnr = (select a#arnr from artbez where
a#fa=999 and a#nl=999 and a#arnr = y.yparnr)