T-SQL loop through all rows in a table to add an incremental value

DECLARE @tempTable table(Id INT NOT NULL PRIMARY KEY)
DECLARE @counter INT
INSERT INTO @tempTable SELECT UserId from TestTable
SET @counter = 1
WHILE (@counter < (SELECT COUNT(UserId) FROM @tempTable)+1)
BEGIN
UPDATE TestTable SET TestColumn = '' WHERE Id = (
SELECT a.Id FROM
(SELECT Id, Row_Number() OVER(ORDER BY UserId) AS 'RN' from TestTable) as a
where a.RN = @counter
)
SET @counter = @counter + 1
END