Absolutely people can change. But it needs to be something you REALLY want. If you're not dedicated to it 100%, you'll fail. I can promise you that.
My best friend used to be very similar to me, he was incredibly shy (In-fact, much more than I am now) terrible around people and used to have really low self-esteem, was quite chubby.
When he was maybe... 16, the two of us joined a group committed to helping youth with problems whether it be depression, homelessness or anything in-between. In that year, he changed almost completely.
When he came out of there 2 years later, he was no longer shy. He was going out to parties, socialising with everyone, and his group of friends increased more in a week than it had the past 10 years of his life, he was fit and in shape and loving the transformation he'd worked towards.
He sought to change something about himself he really didn't like. He didn't like that he was a shy person, he didn't like that he wasn't able to communicate how he really felt.
Now he's the guy I go to when I need something done. I need him to talk to someone for me, he'll do it. I need him to scout out a girl for me, he'll do it.
I've got a lot of respect for him for doing that. He really didn't like something about himself and did everything in his power to change it, and he's never been happier. All I can ever think is "good for him."
Yeah, people can change. Don't expect it to happen overnight, and don't expect someone else to be able to change you - It probably won't happen.
Also, don't expect every change to be a positive one. You need to be careful you're not making a change for the WORSE rather than the better.