have an iPhone 5. am deaf in one ear thus was surprised to find iPhone had incorporated left and right lead volume. am deaf in right ear so turned down to zero to save energy. Now can't find where to change back

I have an iPhone 5. I am deaf in one ear thus I was surprised to find that the iPhone had incorporated a left and right lead volume. I am deaf in right ear so I turned that down to zero to save energy. Now I can't find where to change back? I was bought a new pair of headphones yesterday and it would be nice to share earphones with people. Can't find where to change in the settings