You should have had an extra booklet with the owners manual for the extra functions for Sat Nav and Audio, well that is if the previous owner passed it on. I had all the docco from when my car was delivered including an introductory audio CD.
With the radio on press the left audio round button and a display should come up with 3 upright bars Bass Middle and Treble I think they are called,
Press the round button moves to the next bar and turning the button increases or decreases the effect and is shown by a red line on the bat which moves as you turn the button.
Next is a cross of two bars for right to left balance and forward to read balance which works in the same way,