Ask HN: Can I decline a job offer already accepted?

I accepted a job offer from a US company X, and while waiting for them to send me the contract, I received a better offer from US company Y.

Now I would like to go with company Y, but I already accepted the offer from Company X (I did not sign anything, I accepted it via email).

Is it legal to decline the offer from company X after accepting it? Do I risk anything?