Need Advice!

Hi,

What with only one day to go before I need to accept one of my offers, I was wondering: Am I able to accept a firm + insurance but reject possibly in future. The only reason I ask this is because I feel like I could up my A levels this year and possibly get into a better university. Plus I am still unsure about university.

I have deferred all my entries so if i were to accept won't be attending until next year. If a thought comes to me during the year though, it is possible for me to decline the offer later on? Or do I enter some legal abiding contract whereby my choice is final and it is impossible not to go/apply for another?