You can use a CTE:
;WITH RowNbrs AS (
SELECT ID
, ROW_NUMBER() OVER (ORDER BY ID) AS RowNbr
FROM MyTab
WHERE a = b
)
UPDATE t
SET t.MyNo = 123 + r.RowNbr
FROM MyTab t
JOIN RowNbrs r ON t.ID = r.ID;
You can use a CTE:
;WITH RowNbrs AS (
SELECT ID
, ROW_NUMBER() OVER (ORDER BY ID) AS RowNbr
FROM MyTab
WHERE a = b
)
UPDATE t
SET t.MyNo = 123 + r.RowNbr
FROM MyTab t
JOIN RowNbrs r ON t.ID = r.ID;