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) { }
}