Not sure about that PC to be honest...but if you have access to BIOS options you should be able to OC the CPU I think...?
However prebuilds are very often put together with non standard parts and you can have difficulties upgrading bits because of non standard sizes, small power supplies and missing power supply connections...so it really depends on the exact specs of things like the PSU.....etc. and the space inside the case to accommodate a new GPU...
