• NBT API
• Powerful command API with 1.13+ command completion

To edit an item's nbt the method NBTProvider.getNBTFromItem can be used. This method takes in an ItemStack and returns a NBTCompound instance.

NBTProvider nbtProvider = QAPI.nbtProvider();
NBTCompound itemNBT = nbtProvider.getNBTFromItem(itemStack);
itemNBT.setString("value", "key");
nbtProvider.applyNBTToItem(itemStack, itemNBT);

Changing the NBTCompound instance will not change the item's actual NBT value. To actually update the item's NBT value you must call the NBTProvider.applyNBTToItem method.

Getting an entity's NBT is very similar to getting an item's NBT. All you have to do is call the NBTProvider.getNBTFromEntity method. This method takes in an Entity and returns a NBTCompound instance.

NBTProvider nbtProvider = QAPI.nbtProvider();
NBTCompound entityNBT = nbtProvider.getNBTFromEntity(entity);
entityNBT.setBoolean("Invulnerable", true);
nbtProvider.applyNBTToEntity(entity, entityNBT);

Because this NBT API is just a wrapper around the NBT API already included in bukkit, just multi-version, setting a custom tag on an entity will not work. This is because minecraft does not actually save any entity NBT. For entities, NBT is simply a way of setting variables, like Invulnerability, Speed and so on. If you wanted to store custom NBT data for entities, you would need to do that manually.