I accepted a job offer recently, but have another interview coming up. What do I do?

Company A offered me a position. I tried to negotiate salary but they wouldnt negotiate. Interview with company B coming up, and I know they pay more. If I also get an offer from them, can I go back and try to negotiate even though I signed the offer letter? Also, can I just tell them I change my mind because I received a better offer or is that completely frowned upon?