Create the task and enter a Duration value of 10w. Assign the resource to the task at a Units value of 300%. Set the Task Type for this task to Fixed Duration to "lock" the Duration value of 10w. Apply the Task Usage view and zoom the timephased grid to the Weeks level of zoom. In the timephased grid, you should see 120h of Work assigned to this resource each week over the 10-week time span of the task. Starting in week #6, change the Work value to 80h for each week from weeks #6-10. Voila! That's how to do it. Hope this helps.