Using mySprint S5 on Vacation in Greece?

Hi everyone, and thank you for taking the time to help me.....
I am fairly underachieving with tech aspects of my Sprint Galaxy S5, software version is G900PVPU3BOF6/Hardware G900P.04, I include that info if its needed..

My question is, I am going to Greece in 1 month and want to be able to buy a local SIM card and plug it in and use it with a local number...is it as simple as changing the settings - networks to GSM mode?

I am so confused, reading another thread that says you have downgrade to kitkat,then something about a guy named Odin...

Anyone please,please help me out? I just want to use the phone locally with a local sim card, as I have friends there that I want to be able to call me...

Thank you in advance,

Niko:)