So how can I update my iPhone 6 without a computer

iPhone 6, iOS 12

iPhone 6 is an outdated iPhone it can’t be update to a higher iOS (13 and later).

The last security updates iOS 12.5.7 -> released September 12, 2022, available for iPhone 6.

Check iPhone model & iOS version -> Go to: Settings -> General -> About.

Go to Settings -> General -> Software Update -> Download and Install.

If it is an iPhone 6S you can update to iOS 15.7.

You need about 5 GB device storage and good Service & Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

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