Always take a backup of your whole website (including the files AND the database) before you attempt to upgrade nBill.

When a new version of nBill is released, assuming you have not made any changes to any of the nBill files, you can just install the latest version straight over the top of the old one - you don't have to uninstall first. This is true whether you are upgrading from one version of nBill Lite to the next, from one version of nBill Standard to the next, or from nBill Lite to nBill Standard. In Joomla! this just means installing the new package using the Joomla extension manager. In Wordpress, just upload the contents of the package (ie. the 'nbill' folder) to your /wp-content/plugins directory.

If you are downgrading from nBill Standard to nBill Lite, make sure the version of nBill Lite is at least as recent as the version of nBill Standard you are replacing, and in this case only, you DO need to uninstall nBill standard first, then install nBill Lite (you will not lose any data, as long as you don't delete the database tables). In Joomla! you can uninstall using the extension manager. In Wordpress, you can uninstall by deactivating the plugin from the plugins list, then click the link provided to delete the plugin files (you still will not lose any data as long as you don't delete any database tables).

If you have modified any of the nBill files (for example, by changing the wording of some element in the language files), you can use the manual upgrade patch instead (standard edition only). A manual upgrade patch is always provided with every new release of nBill. You can upload the contents of the patch to your site (using FTP or a file manager), overwriting your old files, then login to the main dashboard in nBill and you will be prompted to click on a link to upgrade your database and add any new elements to your language files. This method will not disturb any language file customisations, nor any custom templates or override files. If you have actually modified any core nBill files though, you will need to re-instate your changes. There is no manual upgrade patch for nBill Lite - if you make changes to the Lite edition, you will need to keep a record of your changes and re-apply them after upgrading.

If you are upgrading from version 2.x to version 3.x of nBill, please be aware that version 3.x no longer has an outright license key option - all license keys are annual. Outright license keys for version 2.x will continue to work in version 3.x until the end of 2015 (there is no fee to upgrade from nBill 2 to nBill 3). You do not have to upgrade to version 3 - if you have an outright key for version 2, you can continue using version 2 indefinitely if you wish (although it will not be kept up-to-date with new releases of Joomla or PHP).