Upgrading osCommerce is actually a little bit complicated for the novice user. The reason is that the upgrade process is different depending on the current version you are coming from. There are no standard steps you can always follow to perform this action, however in later versions of osCommerce there is a file upgrade_guide.pdf included in the installation package. This file contains the information for the upgrade of the version used.
WebHostFace always recommends making a backup of your files and database before performing an upgrade, to avoid any issues that might occur. You can check our great step-by-step tutorial on how to backup/restore your osCommerce.