The Font setting picks the whole notation style: Classic, Pro, Handwriting, Number Notation, Roman numerals, and the Guitar, Piano, and Ukulele chord-diagram fonts.

 

Open Chord Chart Settings (the Aa button on iPhone, iPad, and Mac, or A on Android) and choose a Font. On Windows it is in Settings under General.

 

For more, see Change how your chord chart looks on the iReal Pro website.