It may look easy, but never is
I tend to have new spare securing nuts/studs/ bolts just in case as there is always one stubborn bugger that won't give... and when it does, snaps!
If you have a blow torch, it is a lot, lot easier
Concentrate the heat on the securing nuts until they glow red hot, now spray wd40 on them (you will get a lot of steam/smoke).
You will notice that the nuts will go back to normal colour, but be warned they will be stinking hot still.
Get a socket or decent spanner on them and tighten them a just a tiny bit (cracking on) now give loosening a try ..... If it still won't budge, repeat the process all again.
Lambda's require a special 22mm cut away socket or a long, good quality deep dished goose head ring spanner (to clear the wires and multiplug). Same heating principle if it won't budge.
Important: when re-assembling , DO NOT use exhaust sealing/ jointing paste on pre-cat or pre- lamda sensor joints (before the cat)
. It is safe to use on the other sections of exhaust after the CAT but the chemicals in the paste can react badly with the precious metals inside the CAT & are the quickest way to kill your new CAT or bung up the HG holes in the lambda sensors!
Good luck mate