In the absence of additional information, I must say the odds are that it will overwrite your current install.
You probably should take a look at this:
http://www.ubuntu.com/download/desktop/upgrade
In any event, when you look to upgrade an install, you should back up your current data as if you were reinstalling from scratch, because there is always the chance that something might go wrong. For an extreme example, someone might drive into a light pole causing a power failure in mid-upgrade.
Stuff happens--prepare for the worst and hope for the best.