measurement range on stm32

[Vtrai]

thanks for answering my previous question so according to this i supplied voltages from 0v to 5v and i got following step sizes(if i am correct) what i observed is it stops measuring after 2.5v so does it mean its measuring range is from 0-2.5v / can i connect vdd/vref pin externally to any voltage and can i measure to that extent without damaging the board can any one clarify this to me and thanks in advance and you can find the excel sheet below thanks once again

[Vtrai]

I have gone through the manuals of STM32F4 and understand that VREF+/VREF- determines the measurement range. I have STM32F4 Discovery board for which VREF+ (pin 21) is not exposed on pin headers and seems to be internally set to 2.5 v and is the reason for measurement range of ADC. I want to take full advantage of measurement range from 0 to 3.3V. So, believe VREF needs to be updated (probably to 3.3V?) Kindly help in understanding this measurement range/VREF adjustment (and also impact on other peripherals with modified VREF if I can modify....).