There will be some kind of screw that controls chain tension. Loosen that screw.
The plate that covers the chain wheel will be held in place with another screw. Remove that screw and pull off the plate so that you can now see the entire chain.
Pull off the chain bar and the old chain. Bring the chain to a hardware store and buy an exact replica. Install the new chain around the drive wheel at the back and around the chain bar at the front, making sure the chain bar is properly mounted onto the chain tensioning pin. Make sure the sharp part of the teeth along the top of the chain bar are facing forward, otherwise the chain will be backwards.
Replace the plate and screw it into place until tight. Tighten the chain using the the chain tensioning screw. You want to be able to pull the chain off the bar about 1/4 to 1/2 inch and have it snap back onto the bar.
It is a good idea to sharpen your chain after every 5–6 hours of use. You can buy a sharpening file that matches the size of your chain teeth for about $15.