Greetings,
Welcome to the forum.
I am not a HP employee.
Here is a (Link) to the system board specs; it supports a i5-2400. Are you using compatible RAM?
Carefully examine to CPU socket for any bent pins.
Try removing and reseating the installed RAM with the MB battery removed to, again, reset CMOS.
Are you connecting the four pin EPS power supply connector in addition to the 24 pin MB connector to power the CPU and the MB?
Try a POST (Power On Self Test) with only both PSU power connections and no RAM to see if you can get a BIOS POST error code. It would be impossible to update the BIOS if the device will not POST.
Do not connect any other add-in components or unneccesary peripherals.
Regards