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 , 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?
I have a Discus tank and don't worry about the pH and they do just fine even though they are supposed to be very fussy about pH and generally like lower pH than the water that comes out of my tap. I figured it would be better to just get them used to the default pH level in my water than worry about lowering the pH level of the new water every single time I change the water. As long the pH level is consistant, they do just fine. Something to think about before lowering the pH....
What is your Ph at currently? And why type of fish are you looking to put in your tank? And is your current Ph stable? Sick-lid boy is right on the mark about it being better to have stable Ph and let the fish get used to it then try to adjust the Ph. A small lax in adjustment of the water can lead to a sudden spike which would cause a lot of damage to the fish with stress or even a quick death.
I also agree about having stable pH. My pH is naturally high (above 8) and I've had no problems over the past 2 years keeping fish healthy and happy...although I do stay away from "high maintenance" fish that require soft water and low pH. You may also want to check with your LFS. Most will have RO water for sale to meet your objective in the event you absolutely need stable low pH water for your tank.
MOST fish if properly acclimated will adjust to whatever ph you have coming out of the tap. A fluctuating ph caused by having to constantly adjust it with chemicals is stressful for your fish and they usually come down with some disease and die. a stable ph is much better.
Now that said, driftwood will lower your ph and help it stay stable, certain rocks like limestone will raise it and help it stay stable.