Read the document at https://www.microsoft.com/en-us/windowsinsider/leave-program to know how to leave the Windows Insider Program. The Windows 11 ISO file can be downloaded from https://www.microsoft.com/software-download/windows11; you have to create an installation support (USB or DVD) and perform a clean Windows 11 installation. If your PC has Windows 11 pre-installed, there could be a recovery partition that allows you to revert it back to the factory settings: you have to read the PC documentation to know how to do that.
