No WiFi available, how can I update security?

Here are the steps you have to follow for either updating iOS wirelessly through WiFi or through a computer

Update your iPhone, iPad, or iPod touch - Apple Support

If by security you mean the iOS (iPhone Operating System software), then you do need WiFi or connect the phone to a computer which has internet > launch iTunes > and do the iOS update via iTunes from the computer

Note: when you update the iOS, you are installing the latest operating system designed by Apple for your iPhone and in general overall security of the iPhone is improved.

JoannaSull wrote:

How can I update security on my IPhone without WiFi?

If you mean IOS update, you need to be connected to wifi in order to update over the air.

You don't.