How to decline verbally accepted offer?

I apologize for the wall of text.

 I’ve been working for an MSP that provides desktop support to a large medical facility for two years now. Recently they’ve began the process of moving us contractors over to be full time employees of the health system (Company A). I verbally accepted an offer that I wasn’t too thrilled about since the pay was a very small increase, but the benefits of being with the organization were a no brainer over my current employer. They wouldn’t budge on salary. I verbally accepted an offer from Company A on Tuesday. An hour later I received a phone call for an interview at Company B on Thursday which I accepted to keep my options open. I interviewed and received an offer immediately the next day. My offer letter from A came in the same day as B (Friday). Everything about B is better from 25% more pay, better schedule, and environment. I have decided that I no longer want to work at A and will sign with B. My dilemma is that my manager who is employed by A assisted me heavily in getting the offer, even giving me hints about interview questions beforehand. I feel bad for reneging on my verbal acceptance, but I know this choice will be the best for me. I need to make him aware of my decision to decline. 

For those that have been in a similar situation, what would be the best way to start this conversation, hopefully without burning a bridge? I accept that it may happen, but just looking for advice.

Thanks in advance.

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