Hello,
You can technically cancel or defer a module pretty much right up until a few weeks before the exam. You should be able to cancel your module (or defer it - putting it off until next year, if you think you'd like it better later) by contacting your Student Support Team. You'll be able to find their number on your StudentHome or you can send them a message through it. If you have anything in mind that starts in February 2016, then the registration deadline is currently the 7th of January, so you'd want to have sorted out getting your module changed by then.
I changed from doing MST125 to M140 (Computing & IT and Mathematics -> Computing & IT and Statistics) and it was fairly straightforward. They can give you a lot of advice about your other options as well, and you can speak to an advisor about different modules, degrees, etc.
As the post above says, if you're getting funding, you might need to inform the organisations (SFE, SAAS, etc.) about your degree change.
Good luck!