Unlike opencart 1.5.x, the 2.x version comes with its own extension installer accessible in admin at extension > Extension Installer. But in order to install modules directly from the opencart admin you must have ftp enabled in your admin settings and the right ftp credentials added at the ftp settings in admin > system > settings on the ftp tab.
If you have the ftp set up in opencart then installing modules is very easy. You must simply upload the module archive through the admin extension installer page.The archive you are uploading must have its name ending in .ocmod.zip. If it doesnt have a name like that, when you try to install it, you will get an "Incorect file type!" error.
However if you dont have ftp active and configured for admin or if the extension installer gives you errors while trying to install, you can still install the extension manually via a ftp client like in opencart 1.5.x.
First connect to your store via a ftp client (like filezilla, winscp, cuteftp etc) and navigate to the root of youre store (the root is the folder where opencart is installed, usually directly public_html but it can differ).
Next unzip the archive with the name ending in .ocmod.zip.
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.