okay, not trying to be mean, but you won't be able to do extremely well if you have left it to the last week. if it's not your fault, then don't worry about it as everyone will be in the same boat as you.
i would suggest perhaps watching some freesciencelesson videos, do as many past paper questions as you can etc.
past paper questions are really good as they reuse questions SO much is amazing!
make sure you go over the things that you DONT understand, rather than things that you do.
i hope that helps and good luck x