They are essentially bolt-off, bolt-on units. A friend & I replaced one on his roof a few years ago in about 15 minutes. The hardest part was getting the thing up on the roof, for which we were able to snag a 3rd pair of hands from the neighbor in the next site to assist.
As Don said, just find the part number to get a compatible unit (same BTUs, connections, etc), and it should be a fairly easy install. (I know, famous last words, right?)
One final noteÖ if memory serves, my friend found the best price at etrailer.com. But as itís a pretty common item, shop around.
2013 VOLTAGE 3200 (EPIC I & II)
2014 Ford F-450 PLATINUM
2018 BMW R1200GS RALLYE