I want to format laptop. I don’t want to use windows 8 so I want to change my BIOS from UEFI to legacy. Can you please help me?
I use UEFI to boot occasionally from CD with my ultrabook running Windows 7 Home Premium, and I don't think it's been a problem.
Example on Dell laptop: F2 (enter setup); Boot -- Boot List Option -- Legasy.
As Bruce says, enter the BIOS at startup and change the mode. You will then need to reformat the drive and install your OS(s) afresh.
What OS are you planning on using? If you are going to go with Windows XP or a 32-bit version of a later version of Windows, you will need to change the setting from UEFI to BIOS, but if you are going to use a later version of Windows in 64-bit mode, the change is unnecessary. If you are going to use any of the more recent versions of a Linux distro, it most likely supports UEFI (even 32-bit versions, but they can be flaky), so again, you probably don't need to change it.
If you need to make the change (or really want to), just boot the laptop and hit whatever key is necessary to enter your BIOS setup, look for the setting to change from UEFI to Legacy mode (varies depending on the particular BIOS being used), and make the switch. Save the changes and exti the BIOS and you are done.
Also note that if you do make this change and the hard drive was formatted as a GPT drive, it will need to be reformatted to use MBR. If Windoes disk tools have been used on the drive, it may already have a "fake" MBR on the disk along with the GPT information (hybrid mode). Ideally, you should completely wipe out the first few tracks of the drive at a minimum before repartitioning the drive as MBR (only) and reformatting.