Catalysts increase (or decrease if negative catalysts) the rate of both forward and backward reactions by the same amount. i.e., they catalyze both to the same extent in the same fashion.
So, if Kf is the rate of forward reaction and Kb is the rate of backward reaction, both increase by the same factor.
K(equilibrium constant) = Kf/Kb
The ratio remains the same, hence, equilibrium constant is not altered.