Have you been adding any chemicals to your tank. It's very unlikely that the PH got that high all on its own. How are you testing it (PH probe, test kit brand)? I'd definitely get it tested with another kit or someone else's PH probe to confirm.
If you do indeed have PH that high, I'd suggest going to the grocery store and buy some seltzer water. Try to find some with just water and CO2. Fortunately, that's usually the cheapest brand. Use your discretion if there's anything else listed in the ingredients. Add it slowly, and you'll see results very quickly. If you have or can get your hands on a PH probe, that would be best. I don't know of any good rules of thumb for how much you'd want to add to bring your PH down to normal levels.
Other things that are supposed to work are baking soda and vinegar. Baking soda will raise your alkalinity, so keep that in mind if you decide to go that route.