You can install third-party keyboard apps just like you install other apps on your device. After you install a new keyboard, you need to add it before you can use it. Follow these steps:

Go to the App Store and find the keyboard app that you want. Tap Get, then follow the prompts to install. Enter your Apple ID password, or use Face ID or Touch ID if it's available on your device. Then add the new keyboard. Go to Settings > General > Keyboard > Keyboards > Add a New Keyboard and select the app that you installed. You might need to set the keyboard to allow full access in order to use it.

To delete a third-party keyboard:

Go to Settings > General, tap Keyboard, then tap Keyboards. Tap Edit. Tap , then tap Done.

You can also delete a keyboard by deleting the companion app that you installed from the App Store. This also removes the keyboard from Settings.