How to enable USB Debugging mode on Android devices
(1) The simplest way to enable USB Debugging. Download APK Installer for Android from Google Play.
(2) The other way
For Android 4.x and above
1. Launch the Settings on your Android devices.Tap Developer options
2. Toggle the USB Debugging option.
Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options.
Note: In Android 4.2.2, when you connect your PC to your Android device via USB, Android gets your PC's RSA key (an identifier token). In Android 4.2.2, when you have USB debugging enabled, this now causes a prompt to appear on connection, seen below.
Check the "Always allow from this computer" checkbox and Click OK to accept the dialog.
For Android 2.x
1. Launch the Settings on your android devices.Tap Application
2. Tap Development and Check the box for USB Debugging