How to update my phone if it says it’s up to date

You don't.

What model phone do you have?

• 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 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3.2

If it says that, that means your iPhone is too old and Apple doesn’t support that model anymore!

You can't update something that is already up to date for that device.

If its up to date then you don’t have to update