To remove a Palm application, tap on the Applications button (to bring up the list of applications) and then tap the Menu button and select Delete. Scroll down the list of applications installed that are available for deletion, highlight the application you want to remove and click on the Delete button. You will be prompted for a confirmation. If you tap the Yes button, the application and all of its associated data will be deleted.
The steps to install and remove applications are spelled out in the manual that comes with your Palm OS device. If for some reason you've lost your manual, you can download an electronic version at http://www.palm.com/support/ for Palm, Inc branded devices. Check with your manufacturer (Handspring, Handera, Sony, etc) if you don't have a Palm branded model, most of them also include downloadable versions of the manuals.
If the "application" you want to remove doesn't show up in the list of apps to delete as described above, then chances are its not an application, but a data file. To remove these "files" you'll need to download something like FileZ to remove it/them.