To enchant an item with an enchanted book: left click the enchanted book and drop it on an item that can be enchanted.

To remove the enchants, drag the enchanted item onto a book, or an enchanted book that has enchants that are not on the enchanted item, or are a lower level that the other item.

There is a built in safeguard to prevent players from replacing stronger enchants with weaker ones. If they would like to do that they can simply remove the enchant first, and then apply the weak one.