How can I format PC having fedora 28 as an os ??

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Sorry to hear you want to go back, easy way is to take a Install DVD or other media with Windows and remove all partitions with it and install windows. Since you may want to keep some files do a backup other wise just reinstall Windows it will take care of it.

The easiest way to install Windows again is to use an bootable DVD or USB-stick containing a Windows Image. You can do it via the Microsoft Media Creation Tool etc.

1. Start your PC from the boot media

2. Choose new Installation and let Windows erase all partitions.

3. At this point install Windows as ussually.

If this does not work, erase all partitions manually via Linux Live-CD or Fedora Live-Media and then follow the above mentioned steps. Do mot erase the Windows recovery Partition of your System, you may loose your License, if the windows-key is not stored in a certain UEFI-Region! and your system is delivered in this way.

As far as I know, Windows installations for end users (vs. servers) have a recovery partition which can be booted. If you can figure out how to activate that, typically it will restore the disk to the state it was in when the system was shipped by the manufacturer. It takes a few hours to run but doesn't ask any difficult questions.

For a more experienced computer owner, I'd ask if you looked at the system configuration, specifically the disk partitions, to see if Windows is still present. Maybe all you need to do is change the default boot selection back to Windows.