Yes all i3s are locked .
To overclock an un locked CPU here is a good guide :http://www.wikihow.com/Overclock-a-PC
BUT .... make sure that your Motherboard can handle OC, read the manual or search the model online, look for Power Phase Design, if it's 4+1 or similar then by all means DON'T overclock, it'll fry your mobo
If it's 8+2 then it's safe to OC.
Also check that your PSU have enough power to sustain your PC when OC'ing