How do I decline a job that I've already accepted?

K, so I applied for a framing job with a construction company, and 20 minutes later I got a call saying I'm accepted and can start working in a week.

A half hour later, I get an email from a landscaping company that I applied to yesterday. Turns out that they've accepted me too. I'm much more comfortable landscaping, and I also know the owner quite well, so I'd rather go with Him.

But now I need to call up the construction company to tell them I no longer wish to work for them, and that's stressing me out. It feels weird to go back on my word, seeing as I've already accepted them. Any advice as to what I could say to make the phone call run smoothly?

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