If you are talking about just replacing with a remanufactured or brand new tranny, that in itself is not too difficult, provided you have proper jacking equipment. Preferred would be a special tranny jack but I have heard of people using a regular trolley jack with a tranny adapter. In either case, the critical component is a 4-pointed fork on which you rest the heavy gearbox, which a regular jack doesn't have.
Once you have taken the old gearbox out, you can either take it to a tranny shop to have them rebuild it or try to rebuild it yourself, which of course adds significant complexity. I have heard that shops charge a lot of money to just take it down and reinstall it so you may want to perform just that part and have a professional do the tedious work for you.