If you are trying to say that, you have posted an item, with the "wrong" lot no. And you want to change the lot no. you can do so with "Item Reclass Journals". It allows you to specify a new lot no. (If you are using the warehouse module, and you have enabled lot no. tracking at warehouse level, it gets a little bit more complicated)
If you are saying that, you have posted items without any Lot No. Setup, and now you want to enable lot no. setup, then this is not possible. NAV will not allow you to enable Lot no. when there are existing item ledger entries for the Item. You would have to redo the item again. (This means, creating a new item card, and reentering all the items again with proper lot no. setup)