Assuming a rational transfer function with real coefficients, then this can be done by inputting the complex conjugate of the original transfer function. This can be achieved by multiply each coefficient of odd powers of s by minus one. So for example:
>> figure, nyquist(tf([1 1],[1 1 2 0]))
>> figure, nyquist(tf([-1 1],[-1 1 -2 0]))
yields the same plots, but with the arrows pointing in the opposite direction.