How to Apologize and Reconsider a Job Offer You Initially Declined?

I received an offer to work in one organization, but I declined that offer letter because I received another offer letter from a different organization. However, I am now reconsidering as I am not satisfied with the offer I accepted and am willing to accept the previous offer.

Could you please advise on how to apologize in an email for this situation?

Thank you.

Considering Your Options After Accepting a Job Offer

If you accepted the offer from the second company, how are you going to handle the situation if they don't cooperate and still expect you to start?

Then you need to consider the fact that the first company may not give you a second chance.

Sort all that out before you worry about apology letters.

Whom do you want to apologize to?

I am surprised you are asking for help even for writing an email to say sorry and express your desire to change your decision. No one needs perfect Queen's English in communication. You should simply provide the necessary details and clearly communicate that you will not be joining (or wish to reconsider the previous rejection) and request their cooperation.