On the M Series, hold down the Blue Fn button and # key at the same time. An "A" should appear in the top bar, you can now type letters using the keypad.
On the R Series press the "1-A" button once. This will enable this mode.
Troubleshooting: If letters are not appearing in this mode, only numbers, you may need to update the keyboard app. To do this, open Google Play and search for "gboard". Press the "update" button, to update this app.