Humm.
If the reaction is exothermic it releases heat as reactants --> products.
Which means, if you add heat, the system will want to bring the temperature down, so to speak (i.e. oppose the change). [Le Chatelier's]
Since reactants --> products releases heat, in order to lower the temperature the system will go products --> reactants instead (a backwards reaction)
Like caramel says, this 'uses up' some of the added heat.
So the amount of products will decrease !
for aA + bB --> cC + dD
K = ([C]^c [D]^d) / ([A]^a [B ]^b) (sorry about the crappy setting out)
Products on top, reactants on the bottom.
So less products means a smaller K. (I always get confused with fractions so I sub in random numbers like 1/4 compared to 4/1 LOL ...yeah. << >>; )
& vice versa for endothermic.
Basically, what caramel said except said differently xp
Hope that helps!<3