Given: h2o(aq) + h2o(l) h3o+(aq) + oh–(aq) δh°rxn > 0 when the temperature of a sample of pure water is raised above 25°c,

When Δ H > 0 that means this is an endothermic reaction, and we can assume heat as one of the reactants. SO when the temperature increases so the reaction will move rightward to decrease the reactants and increase the products and as the products increase it will make the Kw increase also as
 Kw = [Products] /[reactants].

So our answer will be: the value of Kw will increase.