To raise your KH quickly you can add calcium carbonate or baking soda, but be careful when adding these b/c sudden changes in the waters chemistry is not good for your fish, add it slowly over a few days. Or you could add some sea shells, coral, limestone or marble chips to your tank and raise it that way, this method is a little more safer IMO and IME. And if your change water's KH is very low compared to your main tank, what you can do is add seashells, ect, to the change water and leave it for a week or so in a bucket to get the KH value close to the tanks, this way you wont have any issues with PH swings.
I will say that unless your fish really do need water that is high in KH ,it is usually not neccessary to change it around. When changing your waters chemistry you should always be careful.