Given that you can't find the correct location to ask: Go to a mechanic to have him fix it for you.
DON'T do it yourself, the chances that you break something or otherwise make mathers worse are way to high.
Pay relatively little now to have it fixed, or need to pay a LOT to get a new gear box and have someone install it. Your choice.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.