I've been told the arches are installed by the delivering dealer so you may find some variance from boat to boat.
While preparing my boat for a 300 mile transport, I found the stainless steel arch on my 2001 H340 was attached with four bolts in each of he four legs and silicone sealer between the arch feet and the deck. Before you start, I recommend getting some 3/8" dowels about 6" long so you can keep the holes in the arch feet lined up with the holes in the deck as you are removing the bolts. I found a little tension in the arch legs as I was removing them. You'll need to fashion some kind of head on the dowels so they don't just fall down below the deck. I tapered one end of each dowel in a pencil sharpener. Two dowels per leg should be enough. They'll be even more important when you re-install the arch.
I recommend removing the starboard bolts first as they're the more difficult. You'll need a helper. Whichever of you is smaller and more flexible should reach down into the battery compartment and then up under the aft arch leg (below the deck) to hold a wrench on the nuts. The other person can then unscrew the bolts from above. I found one of these bolts bent so it never got loose and we had to work it for the entire 3" to 4" length. If the batteries are in the way, take them out. The nuts for the forward starboard leg can be reached from inside the cockpit storage locker. Take everything out and you should be able to get your right hand up to the nuts while laying on your right shoulder. A boat cushion may help avoid some discomfort. While working on these four bolts, I allowed my hips to slip down into the storage locker and got really stuck. My wife had to throw a line over the now wobbly arch and winch me up. It's funny now but wasn't that day. A stack of cushions may help keep your butt from slipping down into the locker.
The bolts on the port side can be accessed via two removable panels in the overhead above the stern berth. It was fairly tight but I had room to put a trouble light in the space between the deck and the overhead liner and could get a wrench up in the space to hold the nuts while my wife unscrewed the bolts from above.
After all the bolts were removed, I tied the arch to the boom to maintain some control and then used a flexible putty knife to carefully break the silicone seal under each arch foot.
Just remember, sometimes you have to go slow and wait for some good luck to catch up with you.
Ron B.