You need to loosen the drivers side pipe from the exhaust manifold. This will allow you enough room to move the y pipe going from drivers to passengers side enough to get the bolt out of the hanger. That will allow it to drop down enough for you to remove and install what you need.
Once you install the new cat, put the bolt back in the hanger but do not tighten down. Align and tighten the pipes to the exhaust manifolds.
Then tighten the hanger bolt.