I don’t know how to download the update

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Nowhere can I find anything that is asking you to update.

But, the update will improve your security while online and add a few new features if you choose to install it.

If you are not interested in the most up to date security and protection, you can ignore the update.

SpycesGh wrote:

Why are you asking me to update the phone?

We are not asking you anything. You are asking us how to update.

Make sure that you have a good Wi-Fi connection

Make sure that the battery is charged to 60%+

On the Home Screen, tap Settings > General > Software Update

If your phone is qualified for an update, it will appear