Usually KH changes are the result of changes to pH and GH. KH is buffering and there is no benefit or need to have low KH unless you are a fan of pH fluctuations.
To increase KH, you can add coral chips to your filteration system. Note that they do not do their job immediately so don't add too much at one go.
For a more immediate method, you can add bicarbonate of soda, this is a powder that you can get from your local grocery store.