I had 2 fox on wire stretchers and thought they were ready to turn. The first one I turned fur out and had no problems. The second one I tried I got halfway turned and put a 10 inch tear from the belly and up one side.

What can I do to save this pelt or isnt there any saving it? Is there a way to sew it and also can I rehydrate the pelt to get it turned around?

Soak the tear to rehydrate it, then stitch it up. Dry it again. A pelt that has over dried a bit before turning can be set outside for a couple hours and it will soften enough to safely turn.

If it has gotten too dry, you may have to wrap it in a damp towel, or better yet put it in damp sawdust to soften it a bit to be able to turn it.

Do I use ordinary thread??

I have my wife sew them with dental floss when I get overexhuberant!

sew it.

learn how to use the stretcher when turning. maybe someone will post a video on it someday.
when turning a pelt, slip the entire pelt up a couple inches on the wire then pull the nose back down and through the wire and then back up the other side. this will have the nbose area fur out. then grasp each hind leg and start to slowly roll the pelt inside out as if taking of a long sock. once you get past the hips you can just pull the entire pelt free from the wire. the section of the nose area will release once the rest of the pelt is turned. i place my foot on the bottom of the wire and just pull up. i have never ripped a pelt since i started turning them this way.
hope that made sense.

The best advice I can give is to not let it get to dry. If it starts to feel papery it is to dry. I tore a few last season and this year I am not letting them get as dry before turning. The amount of time before turning depends on what the area you have your hides in and its conditions. The skin on the bellies is very thin and the first to dry and the most likley to tear.

This is how I turn larger pelts. I take them off the board when they are ready, not too dry. I have a piece of heavy cord about 5 feet long attached to the wall, with a small hook made from a 4 inch nail on the other end. (like and "S" hook with one eye closed to tie it)

Now thread the cord inside the pelt from the rear up to the nose. Turn the nose inward and put the hook through one of the nose holes. Then start gently pulling the hide at the crease until it is fur side out. This method really works well and I have torn a hide using it.

Catch another one and try again!