There is a way and there is a thread that describes it somewhere. But more likely it's not an issue of spring tension but dirty seatbelts. After 15 years or so, the seat belts get stiff from accumulated dirt/dust/body oil...etc.
The thing to do is wash them. I know some guys have washed them in the car and others have taken them out and really went at them with a pressure washer. If you do take them out, disconect the battery first as I've heard that there are little exposive squibs that can be set off accidently if there is power....Idk for sure though.
I don't think I actually tightened up the retraction mechanism and I still have to "assist" the seat belt pull, but I made me feel better for making an attempt to fix the issue.