I did something wrong and just want to rollback a few days.
Is it possible to restore a site in SharePoint Online?
P.S. not the top site, a departmental subsite
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As a short answer, No, you can't
The detailed Answer >>
First, You can't perform a backup and restore in SharePoint Online as we do in SharePoint On-Prem!
So If you need to take a backup, you should
Secondly, In SharePoint online, the OOTB option that might help you to rollback your changes is Saving site as template that only available on the publishing site,
So If you didn't save your site as a template before performing your new modifications, you will not be able to rollback your changes.
By the way, the purpose of
Saving site as templateis not to backup and restore a site, the main purpose is to create a template that can be used for multiple sites with the same features that will accelerate the site customization process!
So you can't overwrite the same site with this saved template, you should create a new site from the saved template. so it's not considered a proper solution.
For more details about save site as template option, check
Restore a Deleted Site
Use the following steps to restore a deleted site from the second-stage Recycle Bin.
Verify that the user account that is performing this procedure has the site collection administrator permission.
In Central Administration, click Site Actions, and then click Site Settings.
In the Site Settings section, click Recycle Bin.
On the Recycle Bin page, select the check box for the deleted site that you want to restore.
To my knowledge Office 365's instances of SharePoint do not allow custom backup/restore procedures.
If it has a strong business impact , you can raise a support ticket to Microsoft , on more thing is that the time of the restore should be chosen based on the last known time in which the item(s) was in the desired state and no more than 14 days in the past.
More information please refer to : https://blogs.msdn.microsoft.com/tomvan/2015/12/23/restore-options-in-sharepoint-online-must-read/