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!

You signed the paper. Unless it hasn’t been processed there is technically an extremely slim chance they could or would stop it in transit. As they say those rounds are down range.

If the extension was for a specific purpose, under a rule of extending specifically to be eligible for a school reservation, or a promotion, or an assignment, and that specific purpose fell through, and of no fault of your own, then yes.

If you just extended for a year to extend for a year, that's your new ETS.

You're obligated but the good thing is you can begin TAP (Transition Assistance) services because you are in the window to do so. At least it buys you time to better prepare whether financially, educationally, or emotionally. Best of luck to you and hang in there!

Why do you think they will entertain a "Do Over", or Whoops I changed my mind? You are adult and made a decision now live up to it.

Years ago it would have been when they had stop loss. Now a days I'm not sure what the policy is.

Once you sign the contract it is done your ETS was in 6 months, you extended for 12 more months, your ETS is now 18 Months. Did you sign the extension so you could PCS back to the states? or so you can go to Korea and come back?
You knew what you were doing when you signed the contract.

During My Last Year Of Enlistment ~ Back When Dinosaurs Were Prevalent, 1964 - 65,
I Was Stationed At Osan AFB S. Korea, & Signed A 12 Month Extension,
Hoping To Reenlist If I Received A Promotion.... WELL, Sh-IT Didn't Happen So I Went To Enlistment Office And Was Told "Tough Sh*t"....
Then I Went To See Our 1st Shirt (1st Sargent) And He Said "Bull Sh*t",
Then HE Contacted The Recruiter..........
I Don't Know Exactly What He Said,
But My Azz Was Out Of There Right On Schedule...
SO... Evidently "YES" !
All You Have To Do Is FIND MY 1ST SHIRT, And He'll Take Care Of It