DonnaD
I'm assuming you are talking about a plugin, right? If not let me know as it matters greatly. Anyways, you put plugin PHAR files in the include/plugins/ folder of your osTicket directory on the server. Unfortunately, we do not own/run/maintain your server so we have zero idea where the previous person installed osTicket on your server. So you'd have to do some digging to find the directory on the server. But once you find the osTicket directory on the server you simply navigate to the include/plugins/ folder within it and place the plugin PHAR there.
Cheers.