Every now and again, Palm Inc. will release a new, and hopefully improved, OS (see Before We Begin). When they do so, you will be notified -- if you registered your Organizer and you still have the same e-mail address. Then all you need do to upgrade your OS is follow the instructions on the e-mail they sent you.
If you didn't register your device, if you are not the original owner, if you changed your e-mail address, or just in case you want to verify that you have the most recent OS, go to their web site, click on Software Downloads, and follow the instructions to upgrade your Palm Pilot's OS. You can find out the Version you have by going Home, Menu, Info, and Version.
While you're at it, download the latest version of the Palm Desktop software too.