UPDATE  tt
SET     [stbk#] = tt2.[stbk#]
FROM    dbo.TestTable tt
JOIN    dbo.TestTable tt2
        ON tt.sttkt = tt2.sttkt
           AND NULLIF(tt2.[stbk#], '') IS NOT NULL

update yourtable
set stbk# = x.stbk#
from (
select sttckt, stbk#
where len(ltrim(rtrim(stbk3)))>0) x
where len(ltrim(rtrim(yourtable.stbk3)))=0
and yourtable.sttkt = x.sttkt