1. Determine what socket the CPU uses. Basically all "U" (like i5-4210u) and "Y" (like i5-4220y) model Intel mobile CPUs uses socket FCBGA1168. "BGA" basically means the pins are meant to be soldered into the motherboard and you cannot upgrade it. All other models like "M", "H", "MQ", "HQ" and "MX" should be socket FCPGA946 which means they actually go into a socket and can technically be upgraded.
2. If the laptop you have does allow you to upgrade the CPU, then you should search to see if there is a version of that laptop with the CPU you want to install. This is because the CPU must be listed in the laptop's BIOS. The BIOS has a list of "acceptable" CPUs. If the CPU is not on the list, then the laptop may not boot up or it may only...