If you are already at a school in Year 10 then I'd stay. moving at this stage it quite disruptive for your studies.
However, if you are determined to move then look at the admissions information on the school's website and follow it. Even if they do not have room, you have a right to be tested and to appeal if they do not give you a place. However, your chances of a successful appeal are quite low and the whole testing and appeal process might well take until Easter by which time it would be REALLY late to change schools.