How can the universe be expanding?
Think of it as a balloon.
It might be easier to understand if you replaced the word expanding with stretching. Imagine what a 2 dimensional universe (length and width only) would be like, basically a flat surface. If you roll up that surface like you would a piece of paper, you will still have a flat surface it will just be curved now. If you continue rolling it until all the edges join, it will be a sphere. Keep in mind the surface of the sphere is still 2 dimensional. If that sphere is then inflated, the surface stretches. It gets larger in the 2 dimensions but it is actually expanding into the 3rd dimension.
So if that sphere can be inflated into the next dimension like if was a balloon being blown up, then a 3 dimensional universe can be "inflated" as well, into the 4th dimension. So the universe is expanding into the 4th dimension. We call this dimension space/time.