To install an application from the PC you need to connect your smartphone to your PC using a USB cable first.
When you connect the smartphone to the PC, you have to switch it to the "USB storage" mode by pressing the on-screen button.
Next, you need to copy the .apk file to your phone's memory card by dragging it to the desired folder. For example, "Downloads".
To disconnect the smartphone from your PC press the "Turn off USB storage" button. The next step is to open "Preferences", then select "Security" and enable installation of applications from unknown sources.
Now you have to open the file manager, select "SD memory card", select "Downloads" (or any other previously selected) folder, find the .apk file you copied from the PC and tap on it. Then press "install application". That's all!
Oh, yes. Be careful with installing such third-party applications on your phone. They're out of Play market's control and may (in some cases) cause data leaks or even hardware damage, so do it on your own risk.
Here's our video tutorial:
See it at our youtube channel.
If you're about to upgrade your phone's firmware, see our How to Update the Firmware on Android guide.