how do you replace battery cables in a 1995 mercury mistique?

Unbolt the old ones and bolt the new ones in the same place. It helps to have the new ones to look at before you start. You can also install universal cable clamps but those are not really meant to be a permanent solution. They're really for a temporary repair but most people leave them on the car.

Both cables will have at least two wires. The positive one goes to the starter solenoid and the smaller one goes to the wire harness or the under-hood fuse box. The black negative cable goes to the engine block and the smaller one bolts to the body. It's REAL important that you don't mix them up. Just change one at a time. Note too that the negative battery post and cable are smaller diameter than the positive one.