I have a 1996 volvo , the driver side seat belt harness is frayed a little.. how much of a safety concern is this in an accident? What is involved in replacing this?

It should be okay in an accident but if you wish to have this replaced it's not a big deal. The big deal will be to find the proper size belt.

If the seatbelt fabric is frayed or severely creased, or even contaminated by a foreign substance, it is compromised and could fail during a collision. You can have that important safety device inspected by your local dealer and you will find an exact replacement can be obtained from them.