Hmm.... The only way to change that is to put the TV into service mode and ajust it. If you are unlucky, this TV might not have a service mode and all settings are controled by little "knobs" spread through out the motherboard of the TV, which would require you to take it apart and tinker with it.
I would recommend you take it to a tv repair place, since they may beable to fix it easily.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix televisions.