Hello guys. I would like to seek for advice here for my scenario.

I am a fresh graduate and applied at 2 mid tiers accounting firm while I have an interview at one of the big4 this Friday. Firm A gave me an offer and asked me to go work tomorrow and I accepted. What if the big 4 or Firm B (better firm with better offer) accepted me next week? What should I tell the HR that I want to resign after 1 week of working? As I know probation notice period is 24 hours. Urgently need advices thank you!