If you want to turn down a job offer, then you should send a formal letter to a company declining a job offer. In the letter you should indicate that you appreciate the offer, and briefly state the reasons for declining a job.
The letter should be addressed to person who offered you the job, and sent within timeframe employer envisaged for your response (usually up to one week).
Regarding possible reasons for declining a job offer, you might explain that you accepted another offer, or that you have decided it was best to stay at your current job. Whatever the case, keep your explanation brief without unnecessary details.
Formal letter to decline job offer template
Dear Ms. / Mr. [Last Name],
Thank you for your offer of employment as [Position name] in the [Division] of [Company name].
I am honoured to have received the offer, because I was very impressed with the company and everyone, I dealt with during the selection process.
However, I regret to inform you that I have decided not to accept the job offer after all. Even though I find your offer as very attractive, I have decided to accept another job offer, which I have received last week. In the beginning of next week, I expect to sign employment contract.
I wish you the best of luck in filling the current position. Please pass along my thanks to the selection panel.
I hope [Company Name] will once again consider me for employment if similar opportunities arise in the future.