Apologise letter for accepting pervious offer which I rejected

Hi I got an offer to work in one organization i declined that offer letter, because as I got another offer letter with some other organisation,

but now I don't like the offer which I accepted . And ready to accept the offer which I got earlier

How to apologise in email

If you accepted the offer from the second company, how are you going to get out of that mess if they don't play ball and 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 apologise to?
I am surprised you are asking for help even for writing a mail saying sorry and wanting to change your decision.
No one needs perfect queen's English in the communication. You should just give the details and communicate clearly that you will not join (or want to take up the one rejected earlier) and request for their cooperation.