I'm afraid there is no such thing as overwound, if the clock is fully wound up and won't go then there is something jammed or broken, it will need to be looked at by a clock repairer, you are welcome to call me and I can try and help.
It is very easy to adjust time keeping on a pendulum clock all you need to do is adjust the nut at the bottom of the pendulum.
Screw anticlockwise (down) too make the clock go slower if is too fast, or
Screw clockwise (up) too make the clock go faster if is too slow.
It is very easy to adjust time keeping on a non pendulum clock all you need to do is adjust the rating lever.
If the clock is running fast move the lever towards the area marked S.
If the clock is running slow move the lever towards the area marked F.
Do this a little at a time and be gentle! if none of this works then your clock might need a clean and service and maybe a new mainspring if it is really erratic.
Please feel free to contact me if you need any advice on this.
The easiest answer to this one is call me and I will try and explain what to do over the phone, make sure you are near your clock or have it to hand.
Other Useful tips can be found at Tony Goodman's Fusee Clock Website