What university is this? A grade in the 70-79 range should be a first, at least at every university I've ever come across.
This'll sound really obvious, but the only way to increase your average is simply to do better in your assessments. If your university operates like any other, the final two years of your course will carry the most weight, so if you get predominantly 2:1s from now on you should come out with a 2:1 average.