How do I change screen timeout on iPhone?

Go to Settings/Display & Brightness/Auto Lock and set the delay you want. There’s also a setting in Settings/Face ID & Passcode/Attention Aware. If this is on then the screen will dim after a few seconds if you aren’t looking at it. It still won’t turn off until the Auto Lock times out.

Adding on to Lawrence’s excellent explanation, keep in mind when you are not looking at your iPhone, it will lock for your security. You can also view the Apple Support Article: Keep the iPhone display on longer - Apple Support