Firstly, know that 5-6 months is a good chunk to revise for Biology.
However, you will need to be dedicated and apply yourself. Check the past papers for the exam board; get the mark schemes too. See which topics continually come up and make sure you understand them thoroughly. Each person is different but I find reading through the textbook and seeking information online to be most helpful. I will take notes and read through them to refresh my knowledge.
It is not going to be easy and you will have to make sacrifices (i.e. staying in and studying instead of social events etc.). But just know that if you are dedicated and work hard you can do it.