I accused him of cheating, how can I say sorry?

How can I tell him I'm sorry for accusing him of cheating when he didn't really do it? I really thought he did because the girl who told me that she witnessed it and she is supposed to be my best friend. The whole two weeks he didn't call me. When I thought he was doing it he was at the beach working and where they were staying they had to call collect, and we don't accept collect calls, my mom or her roommate. How can I show him how sorry I am, because I feel really bad?
I accused him of cheating, how can I say sorry?
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