tsch hey, i also did bio as a higher study subject, just completed units 3 & 4 this year! from my perspective having done units 1 & 2 early, I would say don't focus too much on studying 1 & 2 content, rather focus on units 3 & 4, and try to get a headstart on that if you can.
if during units 3 & 4 you come across content that may link to units 1 & 2, I would say only then revise that part of the content, if that makes sense. not much of units 3 & 4 relates back to units 1 & 2. especially unit 4, it barely covers content from bio 1 & 2. for context in unit 4 you study immunity, evolution, and complete a prac for one of the SACs.
the only links to units 1 &2 for units 3 & 4 would probably be the key science skills, which you might have learnt in other science subjects, bioethics (just try to learn some definitions for this), and the structure of prokaryotic cells and eukaryotic cells, as well as the functions of some organelles (rough endoplasmic reticulum, golgi apparatus, etc).
ik that might sound like a lot, or a bit overwhelming if you're unfamiliar with that content but you can do this!
if you're eligible for doing a higher study subject that must mean that your teachers believe you're ready to do so. at least that's how it works at my school.
If you are still a little bit concerned about having missed content from units 1 & 2, maybe ask your teacher for headstart if they can revise some of it with you. that might help 🙂
