I did mine myself because quotes were like $2500+ for a cheap $350 water heater.
Toughest thing was getting it to the 2nd story attic. I just mounted a harbor freight hoist at the top of the roof/ceiling and used that to lower the old one and lift up the new one.
Other than that, turn off gas, disconnect, drain the old one, take it out, hoist new one, connect everything. Use soapy water to make sure gas is connected right, let it rip.
It is not hard at all.