How to modify the enchantement of an already enchanted item?
Solution 1:
Basically, just enchant the item again with the already enchanted item on creative. You can't double enchant items in survival as far as I know, so this is the command;
/enchant (your username) (enchantment) (level)
Just do that twice, and you should be able to have two enchantments at once! (tested on 1.9)