intel overclock

Hey, I have bought https://www.overclockers.co.uk/8pack-approved-intel-core-i5-6400-4.4ghz-overclocked-quad-core-gaming-bundle-bu-050-as.html , this bundle some years ago, and I am wondering how I can overclock it back to 4.4 ghz. I don't know what I did, but my guess is I re-installed the BIOS...