Firstly, you'd need to get your current college (typically, your Senior Tutor) to agree that migration is a good idea for you. They will want to understand what particular challenges that you are having in their environment, and discuss with you what adjustments or support can be put in place to help you to stay at your current college. However, if you all agree these would not be sufficient (or, if you are certain you wish to change courses and your current course is not offered) then they will generally agree to your migration request. Remember, they are under no obligation to do this, so its best to go into these conversations in a polite, respectful and collaborative way.
If you have any evidence as to why your current college is particularly harmful for your health and wellbeing, this can be helpful to share with them so that they can understand the seriousness of the situation.