The following merge statement replicates text description from the FIRST row in the temmp variable @TextTable to the rows which position is defined in the second table variable @tempTable. From the provided description I did not find an explanation why the value 'ABC' is replicated (and not JKL, for instance) so I assumed that you know in advance which value should be replicated. Therefore the MERGE statement replicates the hardcoded value 'ABC' for demo purpose:

MERGE @TextTable AS target

USING (SELECT Acct, 'ABC' FROM @tempTable) AS source (Acct, TextDesc)

ON target.Acct = source.Acct

WHEN MATCHED THEN

UPDATE SET TextDesc = source.TextDesc

WHEN NOT MATCHED THEN

INSERT (Acct, TextDesc)

VALUES (source.Acct, source.TextDesc);