I have a '84 Jeep Wagoneer. It had some water damage from a leaking windshield, I know the cardboard is damaged

I have a '84 Jeep Wagoneer. It had some water damage from a leaking windshield, I know the cardboard is damaged in several places from previous owner. What's the best way to deal with this?

Hopefully; it isn't damaged so bad; that it has lost its spine (ability to stay in place), which usually doesn't happen unless the headliner board is left outside in the weather for months. If it hasn't lost it's spine; it should be fine to try to replace the material over, most of the time, all that has happened is that it puts a spot on the headliner material, the headliner board is just fine. If it is damaged so bad that you don't want to put new material on it; find a junk yard (salvage yard) and look for a new board.