Go into your device settings - Tap General > Keyboard

Tap Keyboards > Add New Keyboard: From the keyboard menu, take a look at the list of installed third-party keyboards and tap on the name of the keyboard you'd like to activate.

Switching Keyboards: Despite enabling third-party keyboards, the native Apple keyboard will still be the default option. Tap the Globe button to make the switch or Tap and Hold it to select another Keyboard from a list of choices.