For anyone learning Korean, here's a quick breakdown of his letter (as a korean learner myself):

sentence one: finally, my insta's unlocked (cry cry)

  • 드디어 finally
  • 인스타 insta(gram)
  • 열리다 + ~았/었어요 (polite past tense form); to be opened; to be unlocked
  • ㅜㅜ korean texting slang for crying

sentence two: I changed my password a full eight times (surprise)

  • 비번 password, contraction of 비밀번호 (비밀 secret 번호 number)
  • ~만 only (used for emphasis here)
  • [#]번 [number] of times
  • 바뀌다 to change + ~았/었 (past marker) + ~네(요) speaker's surprise observation

sentence three: anyway, I've been kinda meaning to start fresh, so I went and wiped everything clean.

  • 암튼 anyway (contraction of 아무튼)
  • 좀 (contraction of 조금) a little
  • 첨 (contraction of 처음) first/beginning + 부터 from (usually in relation to time)
  • 새 new + ~(으)로 by means of; using
  • 하다 to do + ~아/어 보다 to try [verb]
  • ~려고 to intend to do [verb]
  • 싹 entirely, completely
  • 한번 'once'… however, Koreans often use '한번' to put more emphasis on speech, and it is usually added to the ~아/어보다 form to note that they're trying something out/giving something a chance
  • 지우다 to erase; to wipe clean + ~아/어보다 to try to do [verb] + ~았/었 past tense marker + ~(스)ㅂ니다 deferential conjugation form (more humble/formal than the ~아/어요 form)

sentence four: ARMY, are you all having a good Christmas?

  • 아미 ARMY + 여러분 everyone/plural group of people
  • 크리스마스 christmas
  • 잘 well
  • 보내다 to send (though in contexts of holidays it usually means to have a good time) + ~고 계시다 present continuous (계시다 is the hon. ver of 있다 to have/remain) + ~ㄴ가/나(요) is it that case that [verb]…

sentence five: Eat a lot of tasty food and have a happy Christmas

  • 맛나다 to be tasty + ~(으)ㄴ adjective form = 맛난 + 거 (contraction of 것) thing(s): so… tasty things (or food in this case)
  • 많이 a lot/many
  • 먹다 to eat
  • ~고 clausal connective 'and' used for verbs and adjectives
  • 행복하다 to be happy + ~(으)ㄴ adjective form = 행복한 happy
  • 되다 this verb has so many meanings but in this case it's kind of like something is well/happy + ~시 honorific verb attachment + ~아/어요 polite present tense

You guys can leave holiday messages for BTS on Weverse like:

  • 크리스마스 잘 보내세요! Merry Christmas!
  • 좋은 크리스마스 보내시기를 바랍니다! I hope you have a good Christmas!
  • 새해 복 많이 받으세요 Happy New Year!
  • 크리스마스와 새해를 즐겁게 보내시길 바랍니다! I hope you have a Merry Christmas and a Happy New Year

if there's any mistakes or if you have any questions, let me know (i'm still learning too!)