The buzzing is usually caused by a note not lying flat on the bridge.  This can be corrected either by slipping a narrow strip of paper or thin card between the pressure bar (the pressed steel component that is screwed to the body and holds the notes in tension) and the offending note. This can be done if the note is pressed down with a screwdriver, with the kalimba supported firmly.  If you experiment by placing the small strip either one side or the other, which has the effect of tilting the note, the buzzing should disappear. Alternatively, you could try to twist the note one way or another using a pair of long-nosed piers, testing as you go to hear if the buzzing is diminishing or has stopped. You may have to re-tune afterwards, which you do by sliding the note towards you or away from you (ie. lengthening or shortening it) using something hard like a coin or screwdriver.