It's frustrating when the freshly baked batch of cookies you made turn out hard and dry rather than soft, moist and chewy like you were going for. Whether you made a misstep when preparing the batter or you cooked them for a bit too long, in most cases the cookies are salvageable. While you can't turn them into super moist cookies once they've already been baked, you can add moisture and soften them up enough to be enjoyable to eat.
Wrap or store the cookies while they are still warm. Typically, cookies need to cool completely before storing so they don't become soggy from the trapped hot steam. However, that steam could help cookies that came out of the oven already hard from overcooking. In this case, store them in a sealed container or wrap them individually in plastic wrap while they are still warm. Check them periodically until they have softened to your liking.
Place the cookies in a container with sliced bread. The cookies will absorb the moisture from the bread. Put one slice of bread in the bottom of the container, then add the cookies on top of it. Add another slice of bread on top of the cookies and seal the container. Leave the container to sit for at least a full day at room temperature. The cookies should be significantly softer.
Steam the cookies in the microwave. Dampen a few paper towels with water and wrap two to three cookies in the paper towels. Place the wrapped cookies in the microwave and heat in 10-second intervals until soft. The cookies will absorb the moisture from the paper towels. It is imperative, however, to monitor the cookies closely when in the microwave and take them out as soon as they have softened. Cookies tend to quickly overheat in the microwave, which will make them even tougher, so keep a close eye on them.
Try adding slices of apple into your container of cookies as an alternative to sliced bread to remoisten the cookies.
Avoid using too much flour and overmixing the dough during prep, which can lead to a dry cookie.