Open Camtasia Studio 8
Import you file into Camtasia, then drag it onto the timeline
Select the file on the timeline and then right click it. From the Menu, select Clip Speed...
Changing the Percentage up increases the speed of the clip
Changing the Percentage down decreases the speed of the clip
Press Ok, and than BOOM, the speed of the clip has changed.
Now you can produce your video or do whatever you like with it