Can a 1 year contract extension be canceled?

I'll defer to the experienced reenlistment NCO if I said anything wrong (SFC (Join to see) MSG (Join to see)), but what you are really asking is "my ETS WAS in 6 months, but I extended - can I cancel it?".

IF the contract is sitting on a desk AND hasn't been entered into RETAIN, you could try to get them to tear it up, but likely that won't happen. You entered into a contract and what you're basically asking is "Can I change my mind after signing the contract" (extension in this case)?

IF there were some shenanigans involved (i.e., you can show you were coerced, ineligible to extend, etc), then the extension could be nullified, but after that you'll need to follow the process for requesting you be released from your contract early.

Once that paper is signed and processed, you're in it to win it. Unless you can prove without a shadow of a doubt that the contract was done erroneously. So, in essence, your ETS WAS in 6 months. It's now in 12 months.

Your ETS was in 6 months!
Good luck!