My Ph is too high in my 20 gallon freshwater tank?

[i_love_bugs_8]

It's been a week since i got my three guppies into my 20 gallon freshwater cycled tank. I went to the store yesterday to get my water checked, to get more fish. They said that the Ph was too high and to use the buffer. Other than using the buffer powdery stuff, what can i do to make the Ph go back to normal? The store also said to do a 25% or better, a 50% water change. Is this a good idea? Any advice is greatly appreciated! Thank You!