how to implement non abstract method in derived class

You don't have to - if it isn't an abstract method, then you don;t have to implement it, because it already has an implementation in the base class.
If you want to replace a non-abstract method, then you can "hide" the original by declaring the method as new in the derived class:

abstract class A
    {
    public abstract void xxx(int i);
    public void yyy(int i) { }
    }
class B : A
    {
    public override void xxx(int i) {}
    public new void yyy(int i) { }
    }

Or if it is marked correctly in the base class, you can use override:

abstract class A
    {
    public abstract void xxx(int i);
    public virtual void yyy(int i) { }
    }
class B : A
    {
    public override void xxx(int i) {}
    public override void yyy(int i) { }
    }

There are two cases, depending on the very nature of the base class method, namely if it is:

1. virtual, then you are free to override it.
2. 'standard' (that is NOT virtual), then you could hide it using the new keyword, but it is not advisable (see, for instance: "Is it possible to override a non-virtual method?" at Stack Overflow[^]).