Once the Lead is converted, we cannot unconvert it in Salesforce.

Using View and Edit Converted Leads permission in the Profile, users can edit the Leads. But, users can’t unconvert leads. Even with this permission if the user updates the Lead to different status, ConvertedAccountId and ConvertedOpportunityId fields cannot be updated through trigger. These are system fields.

It is better to delete the opportunity and create new Lead.

Check the below link for View and Edit Converted Leads permission in the Profile.
https://www.infallibletechie.com/2018/10/profile-permission-to-view-and-edit.html

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https://success.salesforce.com/ideaView?id=08730000000ksj0AAA