There are two ways to install an app on the Android TV. Please note that this procedure is specific to MI Android TV. A similar procedure can be used for other Android TV.
Install using .apk file
The first option is to install it from a .apk file. The apk file has to be downloaded and copied into a USB using a computer. Insert the USB drive into TV, go to the location of the apk file inside the USB drive. Open the selected apk file, it will ask you to enable or trust the 'third party app' installation. If you have not done this before, got to settings and enable it. Please note that it may be a security violation if you install from unauthorized source. Before installing an app from an unknown source, understand the consequences and proceed.
Install using an App Store
The second option is to install from Android TV-specific App Store. The App Store has to be installed using the first method - apk installation from USB.
There is one App store called AptoideTV. It has all the major apps which are more than enough to get all the required apps for your Smart TV
Download the apk for AptoideTV-3.2.5.apk from below link. Copy it into a USB drive and insert into your TV. Browse to the location, where the apk is present and click on the apk file. It will ask you to enable Third-Party App installation.
Once the App Store is installed, you can use like any other App Store. Browse or search for an app from different categories and install.
Check out this video which shows how to install the Chrome App on 32 inch MI TV