I want to update my phone version

Since you came from the site describing security on iOS 13.4, Im going to assume you have an iPhone 5S, 6, or 6 plus. These phones cannot be updated beyond 12.5.7, they don't have the hardware needed to run anything higher. If you have an newer model phone, read on -

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.7.6

• If you have an iPhone 8 or higher, or SE 2nd or 3rd generation,you can update to 16.5

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support