Technically, you can still reject the offer as long as you have not signed the offer letter yet. However, it may not be the most professional or ethical thing to do. It is important to remember that accepting a job offer is a commitment to the company and reneging on that commitment could damage your reputation and future job prospects. It is best to think carefully before accepting an offer and to only accept an offer if you are committed to staying with the company. If you do need to decline the offer, it is important to do so in a respectful and timely manner and to apologize for any inconvenience caused.