You will have controls on the monitor itself to alter this display. I think the one you want is "Pincushion".
Press the menu button on the monitor and it will bring up an on-screen display with access to many adjustments. Use one of the buttons to highlight each setting in turn and try to adjust your display from there. If you need a little more help with this, then please post back on this thread and tell me what all the options are in the menu and I will talk you through the settings. j.