iOS 16.1 to move on iOS 8

It is not possible.

Do you perhaps mean how to install iOS 16.2 on an iPhone 8? If so -

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.6

• If you have an iPhone 6S through 7, you can update to 15.7.2

• If you have an iPhone 8 or higher, you can update to 16.2

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