Make sure you do accept the offer in Main Round because it would guarantee you of studying at University this year. If you want to try to see if you can be offered a place of another course (obviously, UAC doesn't consider any other if you get into the a first course), then just go online and change your preferences. So the other courses you had in 3rd, 4th..etc, put them right at the top. Then UAC will consider this new order. If you don't get into the new 1st course you placed, they will keep going.