Sri Sri Ravi Shankar: If a Guru alone could change the karma of a person then he would have changed the karma of everybody in one stroke.
No, you have to do some effort as well.
Guru can definitely relieve you of your sins. When you commit a sin and you realize that you made a mistake, you cannot relieve yourself of sin, but the Guru can definitely do it for you. So in that sense, yes!
At the same time, you have to do Purusharth (self-effort). You need to do some good work. And the sense of love and devotion is equally proportionate to the amount of karma that gets released.