How is the universe expanding if it is infinite?
The universe is expanding in time, not space.
It is a bit of a paradox but you have to take away one dimension from the 3 dimensional universe and add in time to under its expansion.
Next, think of the universe as a ball. We sit on the outer surface of the ball, The ball expands into and from time. That is, objects in the universe were closer together billions of years ago and our "ball" was much smaller then as well. Therefore, as time passes the ball expands.