Basic rule of thumb I like to go by - if the manufacturer's tech specs for the machine doesn't state any other processors (or the one you want to upgrade to), then the CPU cannot be upgraded. As such, the tech specs for the machine doesn't show it to be upgradable for the CPU.
Non-rule of thumb - if you wanted to try it anyway (knowing that it may not work/may ruin unit), then upgrade the BIOS to the newest available, then switch the CPU. It'll either work, or it won't. Or it will work very, very horribly.