I have applied a Salesforce Developer org to learn and test. But there is no easy way to reset to default configurations, it seems to delete the current and apply a new one is the easiest solution.
To delete your developer org, go to administration [Setup] to search for “Company Information”. There is a [Deactivate Org] button in this configuration page.
The permanent delete takes 60 days. You may reactivate within 30 days and contact support after 30 days but within 60 days.
I have click on deactivate on 2018/9/1. I will update information on 2018/10/31.
Amigo's Technical Notes 