How can anything be unentangled?

If you manage to keep the entanglement contained, you can undo it. Imagine an electron in a superposition of $| uparrow rangle$ and $| downarrow rangle$ sent through a Stern-Gerlach apparatus. The spin of the electron is then entangled with its path. $$alpha , | uparrow, A rangle + beta , | downarrow, B rangle$$ If you send the electron through a second Stern-Gerlach apparatus such that the two possible paths merge into the same again, you have erased the information encoded in the position and therefore it is not entangled with the spin anymore.

In reality however, there are many uncontrolled factors which make the information leak out and therefore include more and more degrees of freedom in the entanglement. Hence, if you plan to use the superposition for an experiment you better be fast and have a way to correct for errors.