It's just like any other router.
Basic Setup -> Wireless Settings: Change Wireless Password, and SSID if you like, & Save.
Advanced Settings -> System -> Password Settings: Set new Password & Save.
Dunno what you mean by "security code"! Passwords? There are glaringly obvious options to change those.
There are no individual users of a router; they all share it. You are protecting yourself against outsiders hacking your router.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC