How do I know I my iPhone is factory unlocked

What kind of lock?

There are various types of lock

User Tip by MrHoffman:- How to Unlock an iPad or iPhone? - Apple Community

If you can use the SIM to get data or call service it is enough to know, you don't need to try to update it.

Or look in Settings, general, About and lower down look at the Carrier/Network lock information. If it is unlocked it will say "No SIM Restrictions...."

If you cannot then you need to ask the company who locked it to unlock it. Nobody else can.

If u can able to use any sim card and if you can able to update your phone software means it’s factory unlocked