here are the steps.
1) Backup the Site B using this power shell
Backup-SPSite http://SiteB -Path C:\Backup\site_name.bak
2) run below restore site Bbacommand.
Restore-SPSite http://Home-Path C:\Backup\site_name.bak -Force
1st step will backup the siteb, then during the restore it will overwrite the Home. thats it you donw.
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