Press the number that corresponds with the letter that needs to be entered. Continue to press the key until the desired letter appears.
Alternatively, pop out the keyboard by pressing on the keyboard icon on the top right of the data entry field and toggle between numbers [123], letters - lower [abc] and uppercase [ABC] and symbols [#$%]
If using the Telium line (ICT / IWL) models, press the number corresponding to the letter that is needed then press F until the required letter appears.