SQL & PL/SQL

I need to update one of my tables, and can't seem to get the syntax right. I've tried a 4 different approaches, but not having any luck.  Any help would be greatly appreciated.

Attempt #1: Error: SQL command not properly ended

UPDATE TESTDTA.F4941
SET TESTDTA.F4941.RSDSTN=TESTDTA.F4981.FHRTDQ,TESTDTA.F4941.RSUMD1=TESTDTA.F4981.FHUOM
FROM TESTDTA.F4941
INNER JOIN TESTDTA.F4981
ON TESTDTA.F4941.RSSHPN  = TESTDTA.F4981.FHSHPN
WHERE TESTDTA.F4941.RSSHPN = 647143
AND TESTDTA.F4941.RSDSTN = 0
AND TESTDTA.F4941.RSADDJ  > 110365
AND TESTDTA.F4981.FHCGC1  = 'DIS';

Attempt #2: Error report: SQL Error: ORA-00904: "TESTDTA"."F4981"."FHUOM": invalid identifier 00904. 00000 - "%s: invalid identifier"

UPDATE (SELECT TESTDTA.F4941
FROM TESTDTA.F4941
INNER JOIN TESTDTA.F4981
ON TESTDTA.F4941.RSSHPN  = TESTDTA.F4981.FHSHPN
WHERE TESTDTA.F4941.RSSHPN = 647143
AND TESTDTA.F4941.RSDSTN = 0
AND TESTDTA.F4941.RSADDJ  > 110365
AND TESTDTA.F4981.FHCGC1  = 'DIS')
SET TESTDTA.F4941.RSDSTN=TESTDTA.F4981.FHRTDQ,TESTDTA.F4941.RSUMD1=TESTDTA.F4981.FHUOM

Attempt #3: Error report: SQL Error: ORA-00904: "TESTDTA"."F4941": invalid identifier 00904. 00000 - "%s: invalid identifier"

UPDATE (SELECT *
FROM TESTDTA.F4941
INNER JOIN TESTDTA.F4981
ON TESTDTA.F4941.RSSHPN  = TESTDTA.F4981.FHSHPN
WHERE TESTDTA.F4941.RSSHPN = 647143
AND TESTDTA.F4941.RSDSTN = 0
AND TESTDTA.F4941.RSADDJ  > 110365
AND TESTDTA.F4981.FHCGC1  = 'DIS')
SET TESTDTA.F4941.RSDSTN=TESTDTA.F4981.FHRTDQ,TESTDTA.F4941.RSUMD1=TESTDTA.F4981.FHUOM;

Attempt #4: Error: ORA-00925: missing INTO keyword 00925. 00000 - "missing INTO keyword"

MERGE TESTDTA.F4941
USING TESTDTA.F4981
ON (TESTDTA.F4941.RSSHPN  = TESTDTA.F4981.FHSHPN
AND TESTDTA.F4941.RSSHPN = 647143
AND TESTDTA.F4941.RSDSTN = 0
AND TESTDTA.F4941.RSADDJ  > 110365
AND TESTDTA.F4981.FHCGC1  = 'DIS'
  )
WHEN MATCHED THEN
UPDATE SET TESTDTA.F4941.RSDSTN=TESTDTA.F4981.FHRTDQ
   ,TESTDTA.F4941.RSUMD1=TESTDTA.F4981.FHUOM;