Most of the smartphone manufacturers have their own custom skins for Android and they have different settings. However, the basic steps are the same for all Android smartphones.
Step1: Swipe down from the top of the screen and tap the gear icon in the upper right corner of the screen. Or just go to the menu and tap on Settings.
Step2: Once you’re in the settings, select Biometrics and Security or Security and confidentiality. Some versions of Android may have a lock screen and security but look for the word security in settings.
Step3: Open the security settings and search for Installation of unknown apps or from unknown sources. You can have different titles but you will find this option in the security settings.
Step4: Enable it, you will see the prompt which will explain the risks of installing apps from external sources. Once you press OK , you are ready to install the apps from unknown sources on your Android smartphone.
Step5: Now just tap on the APK file you want to install and continue. Your app may also ask you to give permissions to your storage, etc.