Use stored procedure to get minimum value from column and decrement it and other values
This example code should work, but there are some things to watch out for.
Since this is updating the tables while (presumably) another process is using that table to schedule jobs, concurrency could be an issue.

For example, I'd assume there is another key field not mentioned that the scheduler process is using to identify the jobs. If it's the xNumber/xMachine combination, then changing them could cause an issue.

    CREATE TABLE #xTable (xNumber INT , xMachine INT)

    INSERT #xTable ( xNumber, xMachine ) VALUES (5,1),(6,1),(7,1),(8,1),(9,1),(7,2),(8,2),(9,2)
    SELECT * FROM #xTable AS xt;

;
WITH    cte ( xMin, xMachine )
        AS ( SELECT   MIN(xNumber)
            ,       xMachine
            FROM        #xTable
            GROUP BY xMachine
           )
    UPDATE     #xTable
    SET        xNumber -= cte.xMin - 1
    FROM       #xTable
    INNER JOIN cte ON cte.xMachine = #xTable.xMachine;

    SELECT * FROM #xTable AS xt

    DROP TABLE #xTable;


Initial values:

xNumber xMachine
5   1
6   1
7   1
8   1
9   1
7   2
8   2
9   2


Modified Values:

xNumber xMachine
1   1
2   1
3   1
4   1
5   1
1   2
2   2
3   2