Android will warn about sideloading — tap Settings and allow "Install unknown apps" for your browser or file manager.
Open the downloaded APK and tap Install.
Once installed, tap Open to launch. The app appears in your app drawer.
Sideload builds for testing. These debug APKs are signed with a debug keystore. They are not from the Play Store. Each app collects nothing and sends nothing off your device. Upgrading? Uninstall the previous version first — package names include ".debug" suffix.