Understand that entering the BIOS is system dependent and has nothing to do with the operating system on your computer.
When you boot the system, the first screen usually contains information such as "In order to enter setup" and gives a key to depress, do so, that will get you into the BIOS.
Before turning off hyperthreading, you might want to check with your computer manufacturer as to whether or not this is recommended and other issues it might cause.