Replace exhaust donut on a 1991 Nissan Maxima?

You're going to have to drop both front and rear manifold connections. Should be 3 studs in the front manifold & 3 in the rear. Replace both o ring gaskets while you're in there. If the nuts on the studs are corroded then you might want to take it in to get it done because if you break a stud you will have more problems and higher cost to repair it. If you have accessto a cutting torch heat up the manifold to remove the studs or heat up the nuts as not to break the studs.