Installing fascia boards on uneven roofline
You have a couple options, but only one will make it look truly right without creating an oddly-shaped fascia or a ski jump in your roof. You'll need to raise the rafter tails. They're likely low in the middle because either the wall is sagging or the rafters weren't cut or fit well to begin with.
I would examine how the rafters are fastened to the wall plate. If they're not locked down hard, cut some wood wedges and begin driving them in a little at a time. Set a carpenter's line across the roof edge to act as a guide, and work each rafter up until you have a straight line.
Once everything is in place, re-fasten the rafters in some reasonable way. Hurricane ties might do, or timber screws from above.