Trevor,
There should be a customer-level convergence adjustment option in the menu. No parts required- just your remote control, and a little patience.
If that doesn't do the trick then you may need a service-level convergence adjustment, which can be found in the service menu. The service menus on most models can be accessed via remote control code, but each brand/model will have a different procedure (you didn't say which model you had). In any case, a service-level adjustment should be performed by qualified service personnel because it may require (about 95% of the time) special calibration instruments and techniques not available to the consumer.
Jim Lee- technician, Graham's TV Service (W. Columbia, SC), June 2004