Accpeted an offer and laster that week got a better offer. What to do?

I had received an offer for a 6months contract to full time type agreeement of job which i had accepted and in stage of background check. Received another offer which is 12 months of contract however

I had received an offer for a 6months contract to full time type agreeement of job which i had accepted and in stage of background check. Received another offer which is 12 months of contract however alot better chances of getting full time. I am more interested in this one. How can i figure this one out? Is there a way to get out of accepted offer? Based in ontario canada. Thank you in advance.