How to back out of a job offer?

NTU Undergraduate graduating in May. Applied for many2 jobs but only a few got back to me for interview. At that time, only my previous internship company offered me the offer letter. Told them how much i can delay and they said a week. A week later, no other company got back to me to I signed the offer letter (legally binding) cause I kiasu and worried cannot find job. I feel the pay and job role is meh so I'm still looking for better opportunities as I don't really want to work there but i'm afraid I cannot secure a job after graduating. If a better opportunity comes, how do I tell the HR that I want to pursue other opportunities instead?