Time travel in the Marvel Cinematic Universe can be achieved by utilizing the microscopic universe known as the "quantum realm." The rules of the quantum realm are different than in our universe. In the quantum realm everything is unpredictable with even the flow of time working differently than in our universe.
However, if someone could control the chaos and navigate through the quantum realm, they would be able to enter the quantum realm at a certain point in time and then exit the quantum realm at another point in time. In other words, use the quantum realm as a form of time machine.
This method of time travel would not result in any sort of paradox because changing the past doesn't change the future. If a person travels to the past, that past would become their future and their former present would become the past, which could no longer be changed by their new future.