Here is my scenario.

1. I verbally accepted a job position at company A last week. Company A went on to prepare offer letter and contract. The contract has not been signed by me.

2. I have recently received another job offer from company B, offering better terms with the same amount of job scope.

Since I have already verbally accepted company A, how should I reject them so that I can sign for company B? Will this affect my future/reputation because my promises don't count?

For your information I am in the Software Development line.