Rejecting an offer letter after I've accepted it.?

No, politely decline the first offer "due to altered circumstances", they still have enough time to fill it. However do not do this until you have a firm offer , in writing, for the second job.

I would not recommend the practice as a normal course of business. You can resend your acceptance and work for the other firm. Don't' ever expect to be hired by the firm whom you accepted and then changed your mind. Now, if you have signed a contract, that is an entirely different matter.

Call them. be up front and tell them you were accepted for a job you really wanted and would regret not accepting it, although let them know you were very grateful for their offer and thank them for any time they may feel led to a dead-end. I am sure there are plenty of other people who would do great in the occupation you accepted, so dont feel bad you are only giving someone else a job oppourtunity.

unless you are under signed contract with the first company then no you dont have to take it...write a letter thanking them for the opportunity but unfortunately you accepted prematurely and that another opportunity has come your way that would be more advantageous to you and the achievement of your goals...that having been said you will regretfully decline the employment opportunity previously extended to you

Be courteous - call the person you accepted the job from, and tell them the truth, and that you won't be starting with them. There's no illegality involved, or potential sanctions. It's a little embarrassing maybe, but it happens all the time.

Follow the advice of Whyme. I am in the recruiting business and this happens a lot. As long as your new employer is given enough time to find someone else for the role, you should be fine.

It's not very nice, but since you're fresh out of college, the employer will realize you're not that mature, and that's why you jumped too soon before all your other offers came in. They still have a month to replace you.

no --take the better job