Tap the ♯ or ♭ button right after picking a root letter. ♯ raises the current root by a semitone - C → C♯. ♭ lowers it by a semitone - and you'll see C → B rather than C♭, because Fretscape uses the simpler spelling whenever two names land on the same pitch.
Sharpen or flatten the root
Pick a root letter first (C, D, E, F, G, A, or B), then tap ♯ to make it sharp or ♭ to make it flat. To swap from sharp to flat (or the other way), tap the opposite button. The current accidental highlights next to the letter, so a B♭ chord shows both the B and the ♭ buttons highlighted, and the Building row reads Bb.
Why C♭ shows as B
C♭ and B are the same pitch - what musicians call enharmonic equivalents. Fretscape picks whichever spelling reads more cleanly for chord search, which is almost always the natural-letter version: B instead of C♭, F instead of E♯, E instead of F♭, C instead of B♯.
This usually does what you want. The chord still sounds and plays the same; it just reads as the simpler name.
Picking between C♯ and D♭
C♯ and D♭ are the same pitch too, but neither is "simpler" than the other - they're just two ways to spell the same chord. Fretscape picks one or the other based on your Prefer Sharps / Prefer Flats setting (default: Prefer Sharps).
Open the Settings modal to switch the preference. With Prefer Sharps set, C → ♯ gives you C♯, and typing F♯dim7 keeps it as F♯dim7. Switch to Prefer Flats and the same chords come back as D♭ and G♭dim7. The sound is identical either way - only the displayed name changes.