Step 1 would to reflect on why you got a C / D when you sat Biology and Chemistry originally. As only by understanding why you didn't obtain an A* then, can you understand what needs to change this time around.
Did you put in sufficient effort / time to cover the material?
Did you begin revision in sufficient time?
Did you do plenty of past papers?
Did you drop marks due to lack of knowledge, or poor application of that knowledge?
Were there any eternal factors which impacted your performance?
Do you have any theories as to what might have been the issue?
What were you predicted to get, by your teachers? (This would help us understand whether the C/D grades you obtained were unexpected.)