To perform a transpose on the fly, you will find a toolbar button on the chart preview page.
But the app doesn't know anything about musical keys.
It just shifts the chords by the number of semitones you select in the drop-down list.
So you have to find the right key yourself.
If you are transposing down you might get flats (♭), and if you are going up you might get sharps (♯).
So the obvious thing would be to transpose it up three semitones (like the capo does). But the result looks at least a bit unusal.
To also transpose the name of the key in section titles and annotations, you must include them with square brackets [ ].
See this example in the features section.
Take a look at the user preferences regarding the Musical note B - variation by geographical region.