Baking soda (Sodium Bicarbonate) is cheaper and readily available. Its also easier to control dosage and effectiveness.
Add 1 teaspoon per 20gallons to raise GH (and KH). premix the solution in a container and make sure it completely dissolves before adding to the tank. Wait 6 hours then retest and repeat as needed. KH will stabilize and help buffer your new PH.
to raise GH, use epsom salt (Magnesium Sulfate).
best to test your GH and KH before attempting all this. a KH of 3 - 5 would be a safe range to maintain a somewhat neutral PH.