a-levels to another - but this won't always be possible. your school or college might not allow it, because catching up can be extremely difficult, and until you're up to speed with the rest of the class you won't be able to follow what they're learning and could fall even further behind.
how likely you are to be able to switch depends on how soon you try, the course you want to move to, and where you are studying. talk to your school or college to find out whether it's possible and decide whether it's the right option for you.
another option is to take any a-levels you're missing privately. you could 