How do I remove Insider preview

Leaving the Insider Program and fixing activation are separate tasks, so you should not assume that buying Windows is required. Note your edition, activation error, Insider channel, and build under Settings > System > Activation and Settings > Windows Update > Windows Insider Program. If your channel and core version can move to a retail build, choose Stop getting preview builds and enable unenrollment when the next public version releases. If the installed preview is ahead of every retail path, especially an experimental future-platform build, Microsoft requires a clean installation of released Windows; back up files first because that removes apps and settings. After reinstalling the same licensed edition, connect to the internet and run the Activation troubleshooter. A digital license should reactivate only when it matches that device and edition. If it does not, provide Microsoft activation support with the error and proof of license, but never post the product key publicly.