select A
REPLACE A1 WITH B.Б3 FOR SEEK(A.A1,'B','A1')

select B
SET ORDER TO  0 
select A
SET ORDER TO  0 
SCAN
	select B
	LOCATE FOR B.A1 = A.A1
	IF FOUND()
		select A
		REPLACE A1 WITH B.Б3
	ENDIF
ENDSCAN

* Создаю исходные данные

CREATE CURSOR tab1 (F1 I)
INSERT INTO tab1 (F1) VALUES ( 1 )
INSERT INTO tab1 (F1) VALUES ( 2 )
INSERT INTO tab1 (F1) VALUES ( 3 )

CREATE CURSOR tab2 (F1 I, F2 I)
INSERT INTO tab2 (F1, F2) VALUES ( 1 ,  10 )
INSERT INTO tab2 (F1, F2) VALUES ( 3 ,  30 )
INDEX ON F1 TAG F1
SET ORDER TO  0 

* Делаю замену

select Tab1
REPLACE F1 WITH tab2.F2 FOR SEEK(tab1.F1,'tab2','F1')