I'm not the best at maths. Should I change my dream career of a forensic biologist because of this?

If you are lousy at math how did you get your degree and graduate? Who took your tests for you?

If you were so bad then you wouldn't have earned your degree. Unless you were at the bottom 20 percent of your class then maybe refocusing might not be a bad idea. Only you know what your grades are and where your standing is so you have to make the choice.

There is no punishment for completing an interview and getting offered a job then refusing it. You need interview practice so go and ask questions and do your best. They may not even offer you the job because someone else is more qualified. And if they did offer and you said no thank you the job police do not come beat you. You simply say you can't accept but thank you very much for the interview and offer and wish them well with the candidate they select. Burn NO bridges so stay polite.

Good luck!