They say that Dawkins started believing in aliens and stopped being darwinist? Is it true?
No and no. Dawkins never "started" believing in aliens. He always believed that their existence is more likely than not, given the sheer number of solar systems in the universe where intelligent life could evolve. Moreover, Dawkins never stopped being a Darwinist, as you might guess from the title of his latest book: The Greatest Show on Earth: The Evidence for Evolution
Judging by a Google search for the two words "Dawkins aliens", your story goes back to Creationist quote mining. Dawkins doesn't think it likely
that life on Earth was seeded by aliens. He just bends over backwards to be receptive to the possibility, only to strengthen his point that even if
the story was true, it would be a complement to the evolution story, not an alternative. Alien seeding only begs the next question: How did the aliens come to be? Even if
they, too, were seeded by other aliens still, Dawkins thinks you ultimately have to start with a life form that evolved.
I hope this clarifies Dawkins's position for you.
UPDATE: Here's Dawkins's own smackdown of the story. It is aptly titled Lying for Jesus
. I also find it interesting to learn that it's an old hat:That smackdown dates back to 2008.