I was offered a job and signed the offer letter. However, I haven’t started working yet. Now I’ve found another opportunity and want to decline the first offer. Is it legally acceptable to back out at this point?

Yes, it’s legally acceptable to back out of the job offer before you start working. You haven’t entered into an employment contract yet, so you are not bound by any legal obligations. However, it’s always best to be upfront and professional with the company that offered you the job and explain your decision.