Questions about switching iPhone carrier software

[sideshow23bob]

Hello, I have an AT&T iPhone. It's definitely AT&T and doesn't work on Verizon but the person I bought it from made the Carrier software say Verizon 15.5 (under Settings/General/About). How do I switch this to AT&T? Will putting an AT&T SIM correct this and allow it to work?

Thanks. I'm an Android user and have only done basic web browsing on a past gf's iPhone.

 

[ChAoTiCpInOy]

Is it jailbroken?

 

[sideshow23bob]

I don't believe so, is there a way to tell?

Thanks

 

[ViRGE]

I don't believe so, is there a way to tell?

Thanks

The presence of the Cydia application is generally a dead giveaway.

 

[sideshow23bob]

Thanks for that. I don't see it. I'm going to try factory resetting it as I should have assumed to do from the start, hopefully that'll do it.

Thanks to you both for helping though.