Unfortunately, it's going to take rather a lot of manual painting to fix damage like that. To begin the process of repairing it, you need to reduce the effect of the damage, since I don't think it will be possible to remove it completely in one step. Incidentally, it looks like some kind of water damage.
One method might be to create a curves adjustment layer, fill the mask black, and paint in the curves adjustment using white on the mask. Obviously this is only a first step, further manual painting/cloning will be required.
For example:
