Can i update my iphone 4 to 6.1.3 after its been unlock?

It seems that i could bc i've been using an iphone 4 on T-mobile network and it currently has the OS 6.1.3. But i'm not sure how it was unlocked bc it was given to me like this. I just came in possession with another iphone 4 but its locked from AT&T with iOS 6.1.3. Been trying to unlock it but apparently there isn't a hack for the new OS yet. So anyway around that? Also, for the future.once the phone is unlocked. I can still update to the latest OS?

It's not safe to upgrade stuff. Go buy a new phone!

Well, if you don't see Cydia then it was unlocked via a factory unlock, not using ultrasn0w in Cydia so it should be fine.

If you need to unlock the AT&T network locked iPhone 4, you can try with remote unlocking service. This method of unlocking is safe and simple. It is also a permanent unlocking method. So when you update the iOS in future, It won't get re locked. You can get the remote unlocking service for an iPhone 4 from http://www.simpleunlocking.com/apple-iphone-4/rs16wp9/ at affordable cost. This method of unlocking doesn't require a jailbreak too. You can also visit http://www.mobileunlockguide.com/apple-iphone-4 This site provides free unlocking instructions. This could help you understand remote unlocking service better.

I don't recommend you to update unlocked iPhone, it's not safe to update unlocked iPhone, you may lock your device, it's better to leave it how it's