It doesn't matter which you change; the cutoff frequency follows their product, R*C. Reducing either will increase the cutoff frequency since it's reciprocal. The choice of values is mainly practical; smaller values for R increase current consumption and power dissipation, large values for R increase noise, and larger capacitors are physically bigger and cost more. Since the current values are presumably well-chosen, you could reduce both enough to double the cutoff frequency. Of you could reduce the capacitor alone; usually smaller capacitors are a win, at least down to 100pF or so.