You can change the frequency - i.e. the tuning - of each oscillator by using the three knobs marked "Octave", "Semi" and "Cent".
The Octave knob changes the frequency in steps of one full octave (12 semitones).
The range is -4 - 0 - +4 where 0 corresponds to middle "A" on your keyboard at 440 Hz.
The Semi knob changes the frequency in steps of one semitone.
The range is -12 - 0 - +12, thus equalling one full octave down or up.
The Cent knob changes the frequency in steps of cents, which are 100ths of a semitone.
The range is -50 - 0 - +50, i.e. down or up by up to half a semitone.