WITH t2 AS
(
  SELECT CONCAT(
  RIGHT(LEFT(@AllChars, i1+1), 1),
  RIGHT(LEFT(@AllChars, i2+1), 1),
  RIGHT(LEFT(@AllChars, i3+1), 1),
  RIGHT(LEFT(@AllChars, i4+1), 1),
  RIGHT(LEFT(@AllChars, i5+1), 1),
  RIGHT(LEFT(@AllChars, i6+1), 1)
  ) as token,
  id
FROM (
  SELECT DISTINCT
    ABS(CHECKSUM(NEWID())) % 36 as i1,
    ABS(CHECKSUM(NEWID())) % 36 as i2,
    ABS(CHECKSUM(NEWID())) % 36 as i3,
    ABS(CHECKSUM(NEWID())) % 36 as i4,
    ABS(CHECKSUM(NEWID())) % 36 as i5,
    ABS(CHECKSUM(NEWID())) % 36 as i6,
  Feedbacks.Id as id
  FROM dbo.Feedbacks
) x 
)
UPDATE t1
  SET t1.AppLanding_Token = t2.token
  FROM dbo.Feedbacks AS t1
  INNER JOIN t2
  ON t1.Id = t2.Id;