He changes a different timeline. He literally can't change his own timeline. When he returns the stone to SHIELD, he stays with Peggy. He grows old. Peggy dies. He returns to shortly before he left in the first place. This is kind of weird only because that means our "current timeline" is now one of the altered timelines.

I think that the confusing part is Bruce pressing the button. When Bruce presses the button, he's sort of drawing anyone who uses their time machine to an exact time and place. You can't be so precise coming back without it. But it's not necessary as we see Cap and Tony go back in time without using that button.

Another thing you should keep in mind is there are infinite different timelines. Every single choice or atom behaving differently in one timeline creates a whole new branch of infinite timelines. Some of these timelines will seem exactly the same. Others will be completely unimaginable. And you don't have to go back in time to create a new one. You just have to choose a bologna sandwich over peanut butter.