Changing the head of a lawn and garden string trimmer is much cheaper than buying a new trimmer, and it is a simple fix. You should be able to find a replacement head at your hardware store. If you don't, you can order one online. Universal heads that claim to fit most brand-name trimmers are also available.
Remove the trimmer line spool. Push it in and twist it off on most brands of trimmers.
Use a wrench to remove the bolt that's holding the head of the trimmer in place. Take the head off. On some models you can unscrew the entire head from the pole without removing a bolt.
Place the new head on the pole and put the bolt back in. Tighten the bolt. Twist the spool line back on.