I fully enchanted an item but, I accidentally added smite to it. I want to add sharpness but I can't, because it has smite. Is there anyway to remove smite and add sharpness?
You can use an anvil to combine the enchants of two similar items. When you try to combine two mutually exclusive enchants, the enchant in the left input slot will overwrite the enchant in the right input slot.
To solve your issue you will need to get another sword of the same type with only sharpness on it. Then, put the sword with sharpness in the left slot of the anvil and your fully enchanted sword in the right slot. This should generate a new sword with sharpness instead of smite, while preserving the rest of your enchantments.
Or if you have two smite swords you can combine on a grindstone (village update and on java craftable) and get some XP back or in your inventory crafting put them both in the crafting grid to erase enchants (and combine durability).
The only way I know is to get your enchanted diamond sword and another unenchanted one and combine them in an anvil. However, that will remove all enchantments.