If you got a free 30 day trial version, then it's free for 30 days. They figure that's enough time for you to figure out if it will work for you. If your version was installed 3 years ago, then it won't work and you'll have to ask The Mathworks if they'll give you another 30 day version.
If you got a free version from your university, like the R2014 version my son got from his university, then you'll have to contact your university about their rules for installing it.