My school did this, actually! I was a year 12 at the time, but my friend was one of the year 11s. He did fine, and you'll probably do fine, too - don't forget that next year isn't your chance to do well at specialist, next year is your chance to give it a shot and be prepared for the year after.
What I would do, is get up to date with your functions and algebra sections of methods. Make sure you know the graphs of circular functions sin(x), cos(x), and tan(x). Make sure you know about logarithms and exponentials. Polynomials are neat, but not that big a deal - I'd only bother with simple linear and quadratics (which year 10 usually teaches you more than enough about for you to be fine). Potentially learning about differentiation will also be helpful, but I'd think that learning about those functions is the main thing, because circular functions are a BIG thing in specialist, and exponentials/logarithms come up a LOT in the later differential equations stuff.
The biggest thing I want to mention, though - spec will be teaching you quite a bit in terms of complex numbers and vectors. These are wholely unrelated to methods, and it's likely your teacher will be covering these first. While you're learning about these in specialist, this will be your time to learn things in methods that you don't know that you need to know for specialist. So don't stress too much, I'm sure you'll be fine, even if you don't put a lot of effort into preparing for this