What happens to the balance of reversible reaction at equilibrium when pressure is increased

The equilibrium position shift to the side (reactant/product, depending on the reaction) where there is a smaller no. of moles of gas.

According to Le Châtelier's principle, when a chemical system at equilibrium is distributed by a change in conditions, the equilibrium position will shift in a direction that tends to counteract the change.

Hence, when there is an increase in pressure, the equilibrium will counteract the change by reducing the pressure through decreasing the no. of moles of gas (shifting to the side with a smaller no. of moles of gas).

Note: At constant temperature and volume the pressure of a gas is directly proportional to the number of moles of gas.