Updating an unlocked iphone
Chances are the unlocker used a variation of that method to achieve unlock, apple didn't do factory unlocking when the 3Gs was popular.
If you know for a fact it is factory unlocked then go ahead.
Carrier settings updates can also add support for features like Vo LTE or Wi-Fi Calling.
Carrier settings updates let your carrier provider update carrier network and related settings to improve cellular network connectivity and performance.
On the summary panel, click on the "Check for Update" button then download and install the update if available.
You can disconnect your newly updated i Phone once you are told that it is safe to disconnect.
As new updates are frequently released, you should disable automatic updates in i Tunes if you are using an unofficial unlock.
Check Internet forums several days after the update was released to see what effects the update has on unofficially unlocked i Phones.