I am asked this question all the time. Especially of people genuinely curious if their spouse, child, friend, parent, boss, can change into a good, or at least a better person than they currently are. Here is the reality, we all know people who need to change, and we want them to change ... many times they even WANT to change BUT, often the real problem isn't can people change, it's will they do the necessary work TO change? That's the real question.
My answer to the question would be, "yes people can change! But they must do what it takes to change." And I believe there is really only one primary way. Of course there are many specific ways, but most all ways people change are bascially taken from this one scripture found in Romans 12:2 "Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is--his good, pleasing and perfect will." Essentially what that means is that if you really, I mean really really want to change, then you must THINK new and better thoughts than what you are currently thinking! This would include; changing what you listen to, what you watch, who you hang out with, what you read, where you go, and who you go with, it would also include changing what you say. Then on top of the things you have stopped doing, each of those things would need to be replace with new and better things you are now going to start doing!
We in Christian circles often talk about "crucifying the flesh" which comes from Galatians 5:24 "Those who belong to Christ Jesus have crucified the sinful nature with its passions and desires." Which means that our flesh is going to have desires that are not right, and when we have those desires we can either give into them, which is sin, or we can crucify that passion or desire and live holy. And frankly, sometimes we crucify, sometimes we don't! And I believe it has a lot to do with that word in Romans ... "... be transformed by the renewing of your mind." Many people don't change, or feel, they can't change ... and I think it's because they have not "renewed their mind."
I have heard it said, and I believe it to be absolutely true, "You are what you think about!" If you have an addiction you must think there is a better way to deal with pressure, stress, and/or fear than taking drugs, but until you do believe that, like really believe that, you will keep taking drugs when you are fearful, or stressed.
I have personally seen people change, so I know it is possible! I will close with this last thing, and that is the absolute most important factor in someone changing is GOD! Giving people something worth changing for!