Dear @John Kay,
Thank you for posting your question in the Microsoft Q&A forum.
Based on your inquiry, we understand that you have requested an instruction to delete or rename a sub site collection which accidentally renamed with an illegal character, and you cannot access to this site. We will be glad to assist you in this part.
Rename for SharePoint Online:
You can request your administrator or site owner follow these steps in this article to delete the Sub Site: Delete a SharePoint site or subsite - Microsoft Support
Noted: You must be a site owner or SharePoint administrator to delete sites. If you are a site owner but you are unable to delete a site, your administrator may have turned off the ability to do so.
Rename for SharePoint Online:
- Navigate to the SharePoint site you want to rename.
- Click on the Settings (gear icon) in the top-right corner.
- Select Site Information.
- In the Site Name field, update the name of the site.
- Click Save to apply the changes.
Here are a common error messages when rename in SharePoint you may need to determine: Errors when you rename a SharePoint site address - Microsoft Support
If we are misunderstanding or haven't reach out your main point, please provide us a screenshot so we can troubleshoot your request more effective.
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