Changing University choice

Ok guys. I applied to 3 good universities and 2 safety universities. Now i got offer from one safety university. I am thinking of swapping the other safety university for a good one. It's been 5-6 days since i have submitted my application. How much time do i have left to make this change. I got emails from universities telling me they have started considering my application. Just for info I want to swap uni of portsmouth for king's college for physics. I got 4A*s 6As gcse. 2 As ASlevel and am predicted 3A*s for alevel.