How does carbonic anhydrase change the reaction between carbon dioxide and water?

It catalyzes the reaction by making it fast (both forward and reverse reactions).

Naturally, the reaction between CO2 and H2O is very slow in the formation of carbonic acid. However, in the blood, due to the presence of the enzyme, the reaction is much rapid by millions of factors. This is because the enzyme reduces the activation energy of the reactants. The enzyme is important in respiration of cells and in pH regulation,