First off, it was not an apple, and there’s nothing in the Bible that says it was.
It was the fruit of the tree of knowledge of good and evil.
Next–knowledge is in itself a good thing. It’s a gift of the Holy Spirit. But just as there’s some foods babies are not ready for, so there is some knowledge that the immature are not ready for. And perhaps it may extend to the next life for some things.
In other words, this prohibition (according to Eastern fathers) was not absolute, but temporary. As Adam and Eve grew and matured (for they were not yet in a state of perfection, but of undeveloped innocence), this prohibitioni woud have been lifted.