1. Remove the drive belt
2. Loosen rear roller screws
3. Remove the screw holding the front roller
4. Slip out the roller
5. Slip on drive belt
6. Put back together in reverse order

Please see here for tips and how to replace your drive belt:
http://www.treadmillsrepair.co.uk/2014/08/treadmill-belt-replacement.html

you can also purchase a new drive belt here:
http://www.partsfortreadmill.com/parts-for-treadmill/treadmill-drive-belt/