It can be difficult to save overcooked jam. However, you can try adding in a little water and heating a short time in the microwave to thin it out or by placing it back in the pot with some water. I would start with a 2 tbsp of water at a time over low heat. You just want to heat it up a little so it will soften and become thinner which will allow you to stir in some additional water to thin it out.