how to call non abstract method of abstract class?

Abstract classes are commonly used to abstractly define the composition of an object, just like an interface would.  But, an abstract class also allows you to define default functionality for consumers.

Example:

The System.IO.Stream class is abstract. It implements IDisposable.

Stream.Dispose is a non-abstract method that calls the virtual Close method (not abstract, but it could be in a similar type).

This allows derived types to support IDispose without having to write any code for it, it's already implemented by the base class.

The work of Disposing is done by the Close method.