Insecuring a wireless network means that you make it publicly available which in most cases is called "Free Hotspot". In order to insecure your wireless network you should first log in to the configuration interface using gateway's ip-address in a browser of your choice. In case you didn't set up a username and password, the page should load with all the configurations available.
On the configuration page go to Advanced > Wireless > Security and from the first dropdown menu next to
Security mode: choose None. Restart your router and everyone will have access to your wireless connection.
Please be aware that this action could get your unwanted guests because everyone can access your network and if your computer has services which are vulnerable to attacks, a person could easily get into your computer and gather important data.