If this is the Intel Celeron M @ 1.4ghz, its a laptop and you can't change the cpu.
To be honest, its a bit long in the tooth and if you could change the cpu, it wouldn't justify the cost for any potential performance increase.
Yes you can change the CPU. But the process is difficult and a lot more time-consuming than with a desktop. Not to mention that even if you can
find a compatible celeron M for sale somewhere, the performance increase would be minimal and the price would be ridiculous.
If my guess as to what socket your computer has is right (there are a lot of celeron M's at 1.4GHz, all different we would need more details) then the fastest CPU you can upgrade to is 1.5GHz.