Tables have no order so the only way I can see of ordering you data is by Col1, Col2.
(ie 1,4 will come before 1,11)
If this is OK, then something like this should work:
UPDATE D
SET Col2 = RowId * 5 - 5
FROM
(
    SELECT
        ROW_NUMBER() OVER (PARTITION BY Col1 ORDER BY Col2) AS RowId
        ,Col1, Col2
    FROM YourTable
) D
WHERE RowId * 5 - 5 <> Col2