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Hi. There is a revert option (working for only 30 days) in settings/update-&-securityr/ecovery that allow you to go back to your previous Microsoft OS (windows 8 or 7 depending on what you got before).
If you performed a disk cleanup operation http://winsupersite.com/windows-10/windows-10-post-install-clean
you would not be able to return to your previous OS and require to back up all your data and do a full install again on the previous OS.