Undercooked has a better chance of being rescued than overcooked.
Here is an excerpt from the Stiff Jams or Jellies
page on the National Center for Home Food Preservation
Stiff jams or jellies can be thinned with water or fruit juice. They may or may not form a gel again once they are re-heated, as over-cooking of pectin can reduce or destroy its ability to form the gel structure. You will need to experiment with how much liquid is needed to thin your jam or jelly. It is best to work with only 2 to 3 half-pint jars at most at one time. Try four tablespoons (¼ cup) of liquid for each 8-ounce jar. Over very low heat, melt the stiff jam or jelly in the added liquid, stirring constantly to prevent scorching. When it is all in solution, raise the heat to medium and bring the mixture to a full boil, continuing to stir constantly. Remove from heat and quickly skim foam off jelly if necessary. Fill clean, dry containers for refrigerator storage. It is not recommended to re-process the remade jam or jelly in a canner and store it at room temperature. Jellies and jams thinned in this manner may or may not actually gel, but are likely to provide a mixture that that may spread more easily than the stiff product.