PLSQL UPDATE FROM ANOTHER TABLE

UPDATE ATS_ARAC_TAHSIS ATT set ATT.ID_KBS_SERVIS =(
SELECT a.id_kbs_servis
FROM ATS_ARAC_AKTARIM A 
WHERE  A.PLAKA = ATT.PLAKA 
)

WHERE exists(
SELECT a.id_kbs_servis
FROM ATS_ARAC_AKTARIM A 
WHERE  A.PLAKA = ATT.PLAKA 

)