Can we change a function's implemetion?

There's a framework ,contains 4 functions like:

public long t1(){
    return 10;
}

public long t2(){
    return 20;
}

public long t3() {
    return 30;
}

public long t4(){
    return (t2()-t1());
}

can we change the implementaion of t4 to :

 public long t4(){
        return (t3() - t1());
    }

add: we can't change the implemention of the existing framework.