Cheral,
The default reports should only be cloned, but you know that 😉
The best way to "reset" these is to create a New report of the type you desire. Those are all defaulted. I believe almost every report is set to
Date Range=Past 30 Days
Opp Columns=Hidden (leads only)
no other setup items chosen, but the Leads by Reports will have a Field Grouping already setup for Status or Source
Smart List=blank