Open the Settings app
Tap Face ID & Passcode or Touch ID & Passcode
Enter your current passcode
Tap Turn Passcode Off
Enter your passcode again to confirm
If you mean a Screen Time code, open Settings
Tap Screen Time
Tap Change Screen Time Passcode
Tap Turn Off Screen Time Passcode
Enter the Screen Time passcode to confirm
If you mean a SIM PIN, open Settings
Tap Cellular or Mobile Data
Tap SIM PIN
Turn SIM PIN off
Enter the SIM PIN to confirm