DjNikolaos,

From the default screen
* Press Arranger View (F5). All parts that make up the style selected will be displayed with their corresponding volumes.
* Press (e.g. F4 to highlight LOWER1) and while it is highlighted, use the -BALANCE+ buttons to change it's volume level. This can be repeated for LOWER2 and all the other components of the Arranger.

Thanks,

AJ