I’m not clear how much say an author has when it comes to the cover design of their book. What’s the best way to handle this?

I’m not clear how much say an author has when it comes to the cover design of their book. What’s the best way to handle this?
The winner of the $30,000 prize will be announced on Nov. 8.
The winners will be announced in Montreal on Nov. 13.
Former colleagues remember him as a “a lovely and kind man,” who “could read people by reading only a few paragraphs of their work.”
The two-day literary festival is being launched by The Ampersand Review and the Honours Bachelor of Creative Writing & Publishing at Sheridan College this November.
The winner of the $75,000 (U.S.) award will be announced in Montreal on Nov. 8.