You can find all the moodle versions at: http://download.moodle.org/ If you are on a shared hosting server, then make sure your environment supports all the moodle requirements (which are listed under each version on the above webpage). Like me you may only be able to go to Moodle 2.2.6 since the MySQL version jumps after that and my hosting provider isn't that current.
Unfortunately there are no detailed notes for this process. I'm going through it right now and it has been quite challenging. There is no actual "upgrade" feature. What I'm experiencing is that you need to setup another installation of moodle with the "next" version you are going to (and everyone hesitates on the idea of jumping versions) and then you have to do a backup of your courses, download the zip file, then in the new installation create a course catalog with a temporary course created. In that temporary course, do a restore - select your class zip file and then select the option to restore it to a new course and give it the appropriate name. Then go through your course making sure everything is there and correct; and repeat this process for each class. Don't forget about any add-ons/plugins you may have installed in the older/current version (I initially forgot about 2 and it messed me up initially). Good luck!
Ken has been a great help and resource though.