Changing to flat bars is easy.

Pick a bar that you like. I suggest the widest bar you can find because you are a big guy, you can always cut it down if need be. The wide bar will keep your chest open to breath. Make sure they are 31.8 to match your stem.

Chose new grips. Any mountain bike grips will do but I am partial to Ergon for that added support for your hands.

Brake levers. There are short pull and long pull and they must match your current brakes. Ask any bike shop or tell us exactly what you have.

Shift levers. If you have bar ends, great, purchase Paul Thumbies. Your shifters will attach to these and act as thumb shifters. If not Microshift has nice thumbshifters, just match the number of speeds.

New shifter and brake cables are most likely needed.

It is a bit of a cash outlay at first but think about over the life of the bike and how much more you will ride the bike.

Of course this wouldn't be Bike Forums without members telling you just give drop bars a chance because they are better than sliced bread, and you have more hand positions blah blah blah.

I have converted every bike I have ever owned in the last twenty years, and I haven't looked back. I don't need multiple hand positions, I don't need to be in an aero tuck, I don't need bars that at most are 44 cm wide, I need comfort and I got it. The beauty is if you keep your parts and you hate the flat bar you can always convert back.

Don't even get me started on moustache bars that will most certainly come up.

Good luck and if you have more specific questions just ask.