Auto-lock feature

Hi there!

I can’t turn off my auto-lock function on my iPhone 6 model, since the last 2 ios updates and it is super annoying, because every time I’m using an app, the screen looks itself almost immediately when I’m not touching it for a few seconds. I assume, this feature was deleted through the ios update (because I no longer can find the turn off option), but I’m begging you to bring it back because the experience of being on the phone is just gone without this feature.

Thank you for your help in advance,

Orsolya