BobVila.com

The soft plywood and stained insulation are the sign of persistent moisture, either from a leak, or from condensation. The fact that it is dry at the moment, may mean that you've recently replaced the roof and fixed the old leak. However, some exloration is required before you start fixing things. Is this the only spot that is soft? Do you have adequate ventilation to ensure that the problem is not due to condensation? Are there any other places that show evidence of prior water entry?

Once you've diagnosed the original source of the problem, you can move to the solution. If that's the only spot that's affected, and the source of the moisture is understood, then fix the problem at the source, and then replace that section of plywood. However, I would still suggest you contact a good roofer and have them do a thorough inspection - there may be other things you are not able to see.