In My OpinionI think people do have the ability to change both, themselves and others. Many people feel that cannot change others. At least that's what I see here on EP. It could be we are talking about different things.
For example, I have changed my husband in many ways, but is he what I think he should be? No, but do I think I have helped in making him better, yes. Does he treat me the way I think he should? No, but have I stopped certain behaviors and bad habits that were hurting me. Yes. So even though I influenced changes in him, he is still not the right person for me. We still have many issues but the most destructive things he did are gone because I stood up and stopped him.
From my own experience I can say you can directly influence change in others. That doesn't mean you can make them what you want and they will be the person you wish you had, but even a little is good in my case.
I think this works the other way around as well. I think we can also change people for the worse too. Creating habits and behaviors that are not favorable. That's why we should always choose our friends carefully and with purpose. Knowing that they will and can have a good affect on you and you on them.