When newer version of pt3 come out, you will want to update your copy. There are two versions that are regularly updated - Builds and Betas. Builds are generally more stable and are the "official" releases. Betas are more recent version and usually contain more features and more supported sites, but they are not well tested and can be buggy for some to use. You can switch back and forth between builds and betas, but generally it is advised you use the most recent version of whichever you are using (builds or betas).
To find the most recent versions, please see the PT3 downloads section. Updating copies are WITHOUT postgresql (base installs, or initial installs of pt3 are those WITH postgresql, which you should NOT be downloading if you are reading this). Get the most recently dated download of the version you want (build or beta), and download it.
Once downloaded, insure that you do not have pt3 currently running (including any warning screens you may have up telling you that pt3 needs to be updated). Now run the installer and let it install and you will be good to go. Please note, if you have pt3 installed anywhere else besides the default destination, you must install it where you have pt3 installed currently.