Once something is enchanted you can't reenchant it but you can combine enchantments using an anvil.
Either another pick with Fortune or a book.
You need the same level enchantment in order to combine them into a higher level, two Fortune I make a Fortune II and two Fortune II make a Fortune III.
If you use another pick it must be made of the same material.
It's cheaper (uses up fewer levels) to combine a book and an item than two items but it's easier to get the enchantment you want on an item, but then again if you get an enchantment you don't want it's better to have wasted a book than a diamond pick (for instance).