Not too hard. But invest in a good shop vise and an upper receiver vise block. And get some red locktite for your gas block screws. If they're already loctited on by the factory, you might need a propane torch to heat the block and take them off.
a gunsmith once told me the AR15 is very forgiving to incorrect procedure.
that said, if you want to do it the right way and not risk damaging your upper receiver, get a Geissele Reaction Rod. A vice block will work, but you could warp the receiver or damage the pin index slot when torquing the new barrel.
Also, if using gas block with set screws, use Rocksett instead of red locktite. Rocksett is a ceramic threadlocker with a much higher heat resistance than red and a torque resistance similar to blue. That way if you ever have to remove it, you can.
If you only plan to do this once, just buy another upper already assembled as others have said.