Change it in the bios.
Since it is a BE cpu, you just need to increase the multiplier, but do not just right to 3.8(The 955 BE I had did 3.7 on stock voltage with a coolermaster 212+). You will need to make smaller jumps and test with Prime95. Every cpu is a bit different and not all overclock as well.
You may need to add extra voltage(vcore) at some point(add as little as you can then test).
HWmon if you want to keep an eye on the temps.http://www.cpuid.com/softwares/hwmonitor.html