Hi Graspon
I am Dave, an Independent Advisor, I will help you with this . . .
You cannot disable McAfee and enable Defender, McAfee is so deep rooted in Windows that is not possible to do, the only option would be to uninstall McAfee using the MCPR tool available on their website, then restart and Defender will then be active. You can if you wish later, re-install McAfee if you want to . . .