Hi there,
I suppose you cannot use the media creation tool to revert back to Win 10 stable version. You must use the Clean install the PC with the 21H1 version of Win 10.
https://www.microsoft.com/en-in/software-download/windows10startfres
While Clean install select Custom install, so that the data will not be erased and also regarding the One drive, Yes it will be backed up and you can restore them into your Local PC later .
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