It's a job. The foam gets old and falls apart, trying to re-glue sections will not work as the entire liner needs to be replaced. Fabric stores have replacement fabric in several colors plus cans of fabric glue.

This is a looong video and takes a while to get to the station wagon, which is much easier to work on. The V70 models are easier then the 850s, but still time consuming. Yes, to have a couple of people putting the new liner on is a plus, but a single person can do it: just spray the fabric glue on just half of the fabric, stick the fabric and then spray the remaining fabric and stick the other half. Having 6 inches of overlap is not a waste, it just gives you some wiggle room.
https://www.youtube.com/watch?v=KFz7rvEIFXY

Less than $100 DIY. Upholstery shop cost????

BTW, wearing latex gloves while handling the newly refurbished headliner keeps all of the finger prints off.
--
My back feels better when I sit in a Volvo seat