GW8SP2HP1 Windows client

1) userA create and send Task to the userB. In this task:
Start at 27 Jan 2011 and End at 3 Feb 2011.
2) userB - Accept task..And begin work ...
29 Jun 2011 userB set % =75...
3) 30 Jun The user userA wants to shift date of end of the job for the user userB.
Now time for completion of the job for userB should be 2 Feb 2011.

How it to make userA ?

Serg