Hmmm, you really need an example for this?
Something along the following lines (untested & I'm getting tired):
INSERT INTO DestTable(Field1, Fieldn, SeqNo)
SELECT S.Field1, S.Fieldn, 1
FROM SourceTable S
WHERE [Source record not in Destination table]
UPDATE DestTable
SET Field1 = S.Field1, Fieldn = S.Fieldn, SeqNo = SeqNo + 1
FROM SourceTable S
WHERE S.Keyfield = DestTable.Keyfield
If you haven't even tried to resolve your issue, please don't expect the hard-working volunteers here to waste their time providing links to answers which you could easily have found yourself.