How do I fix leftover lasagna that was slightly dry?

Spread some more sauce over it. Put a pan of water in the oven on the rack underneath while reheating to give some moisture.

Even water, added over the top (a small amount), seal the whole thing with foil, and then reheat at a low temp -- will make a difference.

Use a covered dish in the oven to get some steam going.

If you have any tomato sauce or a can of crushed tomatoes, you can gently reheat the lasagna, with some of this sauce spread over it.