I'm an admissions tutor and I deal with transfers all the time.
The first thing to do is e-mail Exeter and enquire if you could transfer. Don't drop out of your current course or anything. Include your A level grades. You will deal with either the admissions tutor for the course, or someone in the central admissions team.
Their decision will be based on your A level grades and if they have any spaces. Do your A levels meet the standard offer for the course? That's the main criterion I use. You might also have to explain why you want to transfer.
They might set you an offer e.g. if you achieve 60% or more in your current first year. They might let you transfer into the second year, or they might make you start from the first. It depends on the courses and how close they are. You might have to complete a special form listing your modules and grades so far. I've never heard of transfers into semester 2 - this is impossible AFAIK.
If you get an offer, then achieve the grades and you can transfer.
Nothing is guaranteed, and there is every chance they will just say no.