While loop on table rows in sql server

DECLARE @CursorTestID INT = 1;

DECLARE @RowCnt BIGINT = 0;

-- get total count of rows to process

SELECT @RowCnt = COUNT(0) FROM tbl_User_Registration;

WHILE @CursorTestID <= @RowCnt

BEGIN

declare @StrBinary varchar(350)='abc12345'

   UPDATE tbl_User_Registration SET [Password] = @StrBinary WHERE [Password] is null;

   SET @CursorTestID = @CursorTestID + 1

END