Before you proceed it’s better for you to understand what is actually an APK from explanation below:
Android application package (APK) is the package file format used by the Android operating system for distribution and installation of mobile apps and middleware.
The apps come as APK files so that they could be installed on an Android device. Now the thing is, unlike the .exe and .dmg files, these files cannot be installed directly, and we will have to make some arrangements on our Android device first. So let’s see how can you install these APK files manually when you get one someday.
There are many sources of APK files across the web, but be sure to choose a site which you trust. Some APK files may contain malicious software (malware) which could compromise your phones security, so it is best to be cautious before downloading or installing any app from outside of the Play Store (and even in the Play Store you’re not completely safe!). Generally speaking, APK files found on sites like XDA should be safer to download than from elsewhere, but read some reviews and user comments before downloading or installing an APK.
When you want to install an APK manually, just open the APK installer app and navigate to the folder where you have your APK files saved. Now, simply click on the application to bring on the installation screen. Click the install button to successfully install the application.