You use products like
Neutral Regulator to get to 7.0 (if you are high or low) and then you also use either
Acid Regulator to get below 7.0 or
Alkaline Regulator to get above 7.0. There are
other companies that offer products like this, but this just gives you the basic idea of how to do it with additives.
You can also change the Ph simply by adding different things to your tank like
driftwood, seashells, peat, etc. For example, using peat in the tank or in your filter will make the water more acidic. Shells on the other hand will make your water more alkaline. This isn't quite as an exact method as using chemical treatments, but still possible.
Do you mind me asking why you want to change the Ph?