How can a female student balance her desire to switch from studying human medicine with her parents' refusal, which may amount to threats of depriving her of education?

If you can bear the hardship of studying medicine without harm, then complete your studies to gain your parents' رضا (pleasure), for honoring one's parents has great merit. However, if the study is difficult or you fear harm from it, try to convince them to a different major. If they do not permit it, then you are not obligated to obey them in studying a major you do not desire.