@MonikaS9010 To do this you need to check your class name and available instances first . It's throwing error because maybe other table has instances created or new table has created instance. let's follow these steps and you would be able to change the table maping.  

1. Change the class type from abstract to concerete (if it's abstract then only follow this)

2. Then go to DB table mapping rule and change the table name if already there of not then create a new rule and do the table mapping with the class.

3. Save both the instances class and DB table mapping as well.