Can I Un-Reject a guy?

I went to a party last weekend and had two guys flirting with me pretty much the whole night. As I was on my way out, the one I was interested in asked for my number but I did not give it to him, mainly because I felt bad doing so right in front of the other guy who had been pursuing me the whole night (he had also asked for my number already and I declined). If I see him again, how can I let this guy know that I actually am interested? Or should I just let it go?
Can I Un-Reject a guy?
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