Usually you will find only an upload folder but if the extension also includes an ocmod then you will also find an install.xml file.
Open the upload folder and from within it, get the contents and upload them to the root of your opencart store via the ftp client.
Now, after uploading the files, and you didnt find an install.xml file in the archive, you are done, the extension has been uploaded and now you can go to admin and start using it. If in the archive you also found an install.xml file then you must also upload this one. This install.xml file its an ocmod and ocmods, in opencart 2.x, can only be installed trough the admin extension installer. But the admin extension installer only accepts files with name ending in .ocmod.zip (module packages) and .ocmod.xml (ocmods). So go ahead and first rename the file install.xml to install.ocmod.xml.
Now upload the install.ocmod.xml file trough the extension installer in admin. When uploading ocmods trough the extension installer you dont have to worry about ftp settings because the ocmod.xml files get saved in the database.
After uploading the ocmod you must go in admin to extension > modification and at the top of the page click the refresh button. This will refresh the ocmod cache, and it needs to be done each time you install a new ocmod.
Now you are done and you can start using the extension.