Or... Go to the menu, choose sound settings, then go from there.
On the left hand side there are two buttons. The top button is a rocker switch that controls volume. If you want to increase the call volume, click on the top of the button while you are on a call. If you want to increase the ringer volume, you can try turning it up while it isn't on a call. I can't remember if the ringer will adjust that way but I think so. You can definitely change the ringer volume in the settings.