Apple iPhone 4

Find "Passcode Lock"

Press Settings.

Find "Passcode Lock"

Press General.

Find "Passcode Lock"

Press Passcode Lock.

Choose option

Choose one of the following options:
Turn on use of phone lock code, go to 2a.
Turn off use of phone lock code, go to 2b.

2a - Turn on use of phone lock code

Press Turn Passcode On.
Key in a phone lock code of your own choice.
Key in the same phone lock code again.

2a - Turn on use of phone lock code

Press Require Passcode.

2a - Turn on use of phone lock code

Press Immediately or the required interval.

2a - Turn on use of phone lock code

Press Passcode Lock to go back to the settings.

2b - Turn off phone lock code

Press Turn Passcode Off.
Key in your phone lock code.

Exit

Press the Home key to return to standby mode.