I have created a Salesforce Developer org to learn Salesforce functionality. 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 Salesforce developer org, you have to below steps:
1: Go to Setup.
2: Search for "Company Information".
3: There is a Deactivate Org button in this configuration page, click on it.
4: Enter Your Org Name
5: Click on Deactivate Org Button.
First 30 Days: You can change your mind during this time and reactivate the Org.
Second 30 Days: The org is locked and you must contact Salesforce Customer Support to reactivate it.
After 60 Days: The org is permanently deleted and irretrievable.
The permanent delete takes 60 days. You may reactivate within 30 days and contact support after 30 days but within 60 days.