expanium, If you have just bought your player it most likely has the most current version of firmware for your model and you have nothing to gain and everything to lose by trying to upgrade it.
I suggest you enjoy your player as is and if you want to safely change the theme then follow the instructions on How to Modify your Firmware here
http://www.mympxplayer.com/viewtopic.php?t=206
With your type of player it is not possible to adjust the volume without the menu.