Check, if you are looking for this.

DECLARE @TableA TABLE (
	HNum VARCHAR(20), DTG VARCHAR(20), MID VARCHAR(20), filterKey VARCHAR(20), EquipIndex VARCHAR(20)
)

INSERT INTO @TableA
SELECT 'N123','200205291246','6759','----','----'
UNION ALL SELECT 'N123','200205291246','----','999','754'
UNION ALL SELECT 'N123','200205291246','----','999','765'
UNION ALL SELECT 'N124','200205291246','6758','----','----'
UNION ALL SELECT 'N124','200205291246','----','999','754'
UNION ALL SELECT 'N124','200205291246','----','999','765'

UPDATE @TableA
SET MID =  B.MID
FROM @TableA A, (
	SELECT HNum, DTG, MID
	FROM @TableA B
	WHERE filterKey='----' and EquipIndex ='----' and MID <> '----'
) B WHERE A.HNum = B.HNum and A.DTG = B.DTG AND A.MID<>B.MID

SELECT * FROM @TableA