Can I re-take a GCSE subject when I'm at Sixth Form?

So here is the case.
I'm currently first year in Sixth Form and I'm taking English Literature, History and Psychology. Might be quite obvious that I narrowed my subjects down towards Law degree that I planned to take at uni (don't know which one yet). I passed all my GCSE subjects except for triple science from which I've had D. At first I wasn't concerned much because I decided to focus more on other subjects as I thought I won't need science in law, which proved to be quite enormous mistake. I began to enjoy psychology now at sixth form and I know some, but not all unis require a pass at science, preferably biology if I want to take a psychology degree; even work experience for students require a science foundation. In my mocks I usually scored a C or B in biology but because I thought I won't need it at that time, I didn't focus on it much and paid attention to other subjects instead. I know that science is not necessarily essential for psychology, but it would be beneficial to have at least a C in one science subjects, preferably Biology.

So is it somehow possible to re-take the whole GCSE biology course now? If you have any experience or knowledge with such thing, please help I'm stressin...

It is possible to retake, yes. Whether or not you can retake at your Sixth Form depends on the policy of your Sixth Form so I'd suggest speaking to your examinations officer. If you can't retake there, you could look into retaking privately.

So is it somehow possible to re-take the whole GCSE biology course now? If you have any experience or knowledge with such thing, please help I'm stressin...