According to Le Chatelier's Principle," If a system (reaction) at equilibrium is subjected to any external stress, either by changing pressure, temperature or concentration of reactants or products, the system will adjust itself in such a way that the effect is minimized".
So, when more product is added to reaction at equilibrium, the equilibrium will shift in backward direction in order to minimize the effect of concentration of product.
Result:
With the addition of product, the rate of backward reaction will increase resulting in increasing the concentration of reactants.