Comeonecomeall

Hi all, I had an informal interview with a potential supervisor over Zoom after I was accepted to XYZ university. After the interview, I emailed him and restated my interest in working with him. I initially got no response but after a follow-up email, he said that I would be a great fit but couldn't guarantee a graduate position with him. He also stated that I would do well within the university. Since by this time I didn't have any offers ( a week before the deadline), I accepted the offer from XYZ (it was my last choice) as I didn't know of my chances to be accepted at a better school or with a better professor. Recently, I got an offer from another ABC University (ranked higher in MY list) that is a better research fit with more potential advisor options. I am strongly considering accepting the ABC offer and declining XYZ. What will be the most polite and respectful way to inform the professor that I will not be joining the university after all?