1. Open up Catalyst Control Center.
2. Navigate to "AMD Overdrive" This is usually a sub-category under "Gaming Performance".
3. Click on the picture of a pad lock to unlock AMD Overdrive settings.
4. Check the box "Manual Fan Control"
5. Drag the slider to your preferred fan speed.
Edit: Here is a picture
The red box is the lock that you need to click to unlock manual controls.
The blue box is the box you check to control manual fan speed.
Right click image and select "View Image" for a larger image.