How can I upgrade my iOS?

Just to clarify, since you posted from that specific support document (About the security content of iOS 12.5.6), an iPhone 12 mini as shown in your tag line, will not get iOS 12.5.6

Its current version should be iOS 15.6.1

As mentioned, you can get it from the Software Update section in Settings->General

You can update over WiFi in Settings>General>Software update or you can use a PC or a Mac. See this for more information.

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

settings -> general -> updates

or by iTunes using computer device