My work were using Opencart for an E commerce site when we came across a flaw in one of the plugins, so I slightly changed the plugin as the original did not capture the start date and some cards (in UK anyway) need that feature.
I did not write the origional, but it is freely available via the Opencart website under the same name.
##Install## If you already have this installed then you should remove and uninstall the origional files if possible. Although no actual database information was changed or altered so a replacement of origional files should be possible.
BACKUP YOUR FILES BEFORE YOU ATTEMPT AN INSTALL
Upload like any other plugin and it should just run as you would expect.