Sony Experia Z2 upgrade from 5.0.2 to 5.1.1

Hi There
I am from Bangladesh & seeking your expert help
My friend gift me a Sony Experia Z2 phone (D6503).
It was a unlocked USA Carrier phone.
Software version is showing 5.0.2 & when i check for update, it says "your device is up to date"

Why ?

How do i understand, my phone is rooted or not?

If rooted, any problem/issue?

Shall I wait for automatic update ? or I have to root?

Kind Regards
Timothy