a.t, I won't be the most updated person on the latest Psychology syllabus, seeing as I haven't done Psych for 3 years and last I heard, they did a massive overhaul soon after I did it.
As far as I can remember, most of Unit 1 was really about research methods (so how to write a psychology report, how they research, what sort of stuff psychologists do, what psychology really is, etc.), psychological development and a few more topics that I forgot.
Most of the stuff you probably need to focus on though is the research methods. (According to the study guide, it doesn't look like research methods is on there, but I only skim read it, so maybe I'm missing something here.) It looks like you should focus on knowing the intrinsic side of psychology (brain, brain functions, etc.), children's psychological development and several of the big debates, which should give you more insight to another side of psychology to the one you're currently learning in Unit 2.
As I said, I'm not updated, so I think your best bet it to read through the
2016 Psychology study design for unit 1 (see pages 14-19 of link) and then follow up on that with the appropriate reading and research. If you want to double check, I'd advise you see your teacher, particularly for Outcome 3 as that will vary from school to school, and see how you can catch up.
I apologise for not being able to help you much.