public class Test2
{
    public static void main(String[] args) throws Exception
    {
        // editing method annotations
        {
            Method targetMethod = Test2.class.getDeclaredMethod("foobar");
            final Something oldAnnotation = (Something) targetMethod.getAnnotations()[0];
            System.out.println("oldAnnotation = " + oldAnnotation.someProperty());
            Annotation newAnnotation = new Something()
            {

                @Override
                public String someProperty()
                {
                    return "value2";
                }

                @Override
                public Class<? extends Annotation> annotationType()
                {
                    return oldAnnotation.annotationType();
                }
            };
            Class<?> executableClass = Class.forName("java.lang.reflect.Executable");
            Field field = executableClass.getDeclaredField("declaredAnnotations");
            field.setAccessible(true);
            Map<Class<? extends Annotation>, Annotation> annotations = (Map<Class<? extends Annotation>, Annotation>) field.get(targetMethod);
            System.out.println(annotations.get(Something.class));
            annotations.put(Something.class, newAnnotation);
            System.out.println(annotations.get(Something.class));

            //targetMethod = Test2.class.getDeclaredMethod("foobar");
            Something modifiedAnnotation = (Something) targetMethod.getAnnotations()[0];
            System.out.println("modifiedAnnotation = " + modifiedAnnotation.someProperty());
        }
        // editing class annotations
        System.out.println("====");
        {
            final Something oldAnnotation = (Something) Foobar.class.getAnnotations()[0];
            System.out.println("oldAnnotation = " + oldAnnotation.someProperty());
            Annotation newAnnotation = new Something()
            {

                @Override
                public String someProperty()
                {
                    return "another value";
                }

                @Override
                public Class<? extends Annotation> annotationType()
                {
                    return oldAnnotation.annotationType();
                }
            };
            Method method = Class.class.getDeclaredMethod("getDeclaredAnnotationMap");
            method.setAccessible(true);
            Map<Class<? extends Annotation>, Annotation> annotations = (Map<Class<? extends Annotation>, Annotation>) method.invoke(Foobar.class);
            annotations.put(Something.class, newAnnotation);

            Something modifiedAnnotation = (Something) Foobar.class.getAnnotations()[0];
            System.out.println("modifiedAnnotation = " + modifiedAnnotation.someProperty());
        }
    }

    @Something(someProperty = "some value")
    public static class Foobar
    {
    }

    @Something(someProperty = "value1")
    public static void foobar()
    {

    }

    @Retention(RetentionPolicy.RUNTIME)
    @interface Something
    {
        String someProperty();
    }
}