I want to update my iOS 14.8 to ios 15

See the Apple articles below:

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

https://support.apple.com/en-us/HT204051

Recommended settings for Wi-Fi routers and access points

https://support.apple.com/en-us/HT202068

Tap on Upgrade to iOS 15.

You do have the option shown at the bottom of the screen.

However, as the message indicates, you will need to connect to a Wifi network.