If you've opened ArtRage and it auto-checked for updates (or you manually went to "Help → Check for Updates") it's found that there was an update, you should be brought to the Members Area page. Log into the Members Area and you should see your registered products listed. Click on the download link for AR5 (it will be the latest release). Once downloaded, double click the file to install it (you don't have to uninstall the older AR5; it can be installed over it).