I am a little confused, you do realize that you are already on UEFI? Legacy is not available on the newer systems. The following is from Dell support pages: Note: Some newer computers will only support "UEFI." It will not include an option for Legacy at all even with the latest BIOS version installed. This is normal, and it is working as expected.
  أهلاً  لماذا لا تختار UEFI أو Legacy؟ [IMAGE]       [IMAGE]
How to change Legacy Bios to UEFI bios