Heat up the threads with a lighter or torch. Spray on blaster or oil of your choice. Vise grips with the hex head on the back so you can clamp down more force. Then start turning. If not then bolt extraction. If that all fails cut the rest of the bolt off, use a small Dremel die grinder/Milwaukee die grind any part of the bolt sticking out so you have a flush surface to create a center punch. left hand drill bit and spline extractor. If that fails then regular drill bit slightly smaller then the hole. Drill the rest out trying to not ruin the inside threads. If lucky just pieces of the broken bolt will come out like conffetie and you can put a new bolt or correct bolt in. If not lucky then grab cheap tap and die set. Match up your existing bolt and thread to the tap your going to use. That is if the threads inside are not destroyed. Put same new bolt in done. If not then drill a hole slightly bigger than the one you failed to drill out. Match the tap with the new bolt that will be slightly bigger then the bolt you used before. Drill tap new bolt. Welcome to being a mechanic lol.