Is it really safe for my iphone 6 (semi factory unlocked) to update to the latest ios?

I was told that something could go wrong with the sim if i try to update it but i've also saw others saying that it was safe so i'm currently in a pinch right now. What are the risks?

As long as it's not rooted, any update should be fine.

My iphone 6 is running iOS 12.1.3 perfectly.

Just make sure you have plenty of free space - several gigabytes; most update problems are due to the phone storage being near full so there's not enough free space for the update download, unpacking and other temporary files needed.

If you have any doubts,
take it to an authorized iPhone dealer and have the update done there.