Effect and Safety of Semaglutide 2.4 mg Once-weekly in Subjects With Overweight or Obesity Who Have Reached Target Dose During run-in Period

This study will look at the change in participant's body weight from the start to the end of the study. This is to compare the effect on body weight in people taking semaglutide (a new medicine) and people taking "dummy" medicine. In addition to taking the medicine, the participant will have talks with study staff about healthy food choices, how to be more physically active and what a participant can do to lose weight.The participant will get semaglutide for the first 20 weeks. Then the participant will get either semaglutide or "dummy" medicine - which treatment the participant gets after the 20 weeks is decided by chance. The participants will need to take 1 injection once a week. The study medicine is injected with a thin needle in a skin fold in the stomach, thigh or upper arm. The study will last for about 1.5 years.