Click on the knobs and drag up or down to adjust their value.
The 'Mid' knob is more precise than the other knobs (it's harder to move it). It is set for fine tuning.
The 'Treble' knob has limited movement range, which can be specified (or omitted) in the options object's bounds object left and right properties.
To fine tune the knob values, hold ctrl (option on Mac) while adjusting the knob value.