I didn't do access to science but I did do access to engineering which shared a lot of similar units.
Things I would brush up on are;
Basic maths (addition, subtraction, division, algebra)
Biology (Cellular mitosis, osmosis, respertory system, nervous system etc)
Chemistry (basic reactions and how the periodic table will help immensely)
Physics (re-familiarise yourself with netowns laws of motion and the basics of electricity)
I'm currently studying Access to Science that I started in January, so we're already halfway through the course (currently sitting on 24/24 credits at Distinction)
Some of the modules that we've covered:
Biology: Nutrition, Digestion and Excretion, Genetics (DNA Replication, Mitosis, Meiosis, Monohybrid/Dihybrid crosses), Skeletal structure/system, Cardiovascular system/structure, Respiratory system, Infectious disease.
Chemistry: Moles of substances, Atomic Structures, Bonding - Ionic, Covalent, Metallic, and Crystal Structures, Periodic table, Oxidation and Reduction.
Physics: Physical quantities and forces, Properties of matter.
Maths: Algebra, Logarithms and Functions, Polynomials, Probability, Statistics.
You should e-mail or ring your college and ask for the list of units that your Access to Science course covers a lot of them are structured differently. Invest in AS and A2 revision books as they have helped me tremendously since I started, and take advantage of the library at your college.