Reject a job after accepting it?

I was interviewing for a few companies and landed a job with one of them. I accepted the offer thinking it’s the only one I’ll get. Joining date was a while away and during that I landed a job at an even better company. I’m planning to reject the first one, and go ahead with the second one.

How to do this in the best possible way and what are the repercussions of this?

Thanks!