Assuming you're using the LG keyboard, go to Settings, Language & Input, then tap the gear next to "LG Keyboard". Go to "Keyboard height and layout", then "QWERTY layout", and you'll see the buttons that you can drag around, to rearrange their locations on the keyboard, or to add/remove them from the keyboard.
Drag the gear icon onto the keyboard, if it's not there already, then on the popup that appears, select that you want it to be a Microphone button.