Replacing a class on JVM startup

Question

Java

There is some java application, let's call it A . Application A consists of just one library.
Our library B extends the functionality of application A by replacing some of its classes with our own.
Currently, you have to manually replace the classes in library A with extended classes from library B .
Question:
How can we force the JVM to use classes from our library B without having to manually replace classes in library A ?