Why abstract allow the modification ? why interface does not allow the modification

Where did you get the idea that abstract classes could be modified but interfaces can't?
The difference is that interfaces can define no code at all, they just set out the signatures that a calls must implement in order to add the interface. Interfaces are not classes, they are a "membership contract" which when followed allows the immplementing class to become a "member of the interface's club".
An abstract class can define code, which can be overridden in derived classes. An abstract class is a full class, with the proviso that an abstract class cannot be instantiated: you cannot new the new keyword directly on an abstract class.

The difference allows a class to be derived from - for example - Control, but also implement an interface such as IEnumerable which permits it to be used in a foreach loop.

Abstract classes and interfaces are not the same, and neither can be "modified" without changing the source code for the abstract class or interface.