Open Control Center and tap the Focus icon
Tap Sleep to turn it off
Open the Health app
Tap Browse
Tap Sleep
Tap Full Schedule & Options
Turn off Sleep Schedule if you want to stop it from turning on automatically
Open Settings
Tap Focus
Tap Sleep
Turn off Sleep Focus
Open Settings
Tap Display & Brightness
Tap Auto-Lock
Choose a longer time or Never
Press the side button to wake the iPhone if it is only dimmed or locked
Restart the iPhone if Sleep mode does not turn off normally