It’s not an an extension “letter.” Letters of extension are for evals, and are completely unrelated.
If your extension has gone operative (meaning that you are already inside the extension), then no, you can’t shorten it.
”Extension becomes operative” or “operative date” refers to the date the extension commences, which is the date after expiration of enlistment, as extended, or as adjusted for the purpose of making up time not served. Following execution and prior to this date, the extension will be considered inoperative.
Note: Once an extension agreement becomes operative it may not be cancelled. In order to alter or cancel an operative extension, Sailors must submit a petition to the Board for Correction of Naval Records if they believe the extension was executed erroneously or that justifiable cancellation adjudication did not occur prior to the extension becoming operative.
-MPM 1160-040, bottom of page 2, top of page 3.
Also: I know that people always say this, but you DO NOT WANT TO GET OUT RIGHT NOW. SELRES is slammed for quotas right now, and the economy is megafucked. If you think you’re going to be fine on the outside, please check, doublecheck, and triplecheck your plan. Your dad’s uncle’s neighbor’s buddy who offered you that job last thanksgiving might be dead or bankrupt now.