Describe how an increase in temperature affects the equilibrium of the main exothermic reaction of the Haber process?

Firstly you must recognize that the reaction that is referred to is the one where nitrogen combines with hydrogen to produce ammonia: N2 + 3H2 --> 2NH3. When talking about changing conditions to a system in equilibrium, one must refer to Le Chatelier's principle that states that if the conditions for a system in dynamic equilibrium changes, the equilibrium shifts to counteract this change. First let us deal with temperature. Your are told that the reaction is exothermic therefore the forward reaction produces heat. Hence if you increase the temperature the equilibrium will shift to the endothermic side (the left) in an attempt to decrease the temperature.