Can I uninstall the iOS 26.2 update on iPhone?

no, that is not possible

other than that you can limit the glass stuff a bit by reading

Hate iOS 26's Liquid Glass Design? Here's How to Tone It Down - MacRumors

you can complain to apple using their feedback channels

Product Feedback - Apple

or reach out to their support channels using

Get

Support (apple.com)

And

or

Contact

- Official Apple Support

And

or

Contact - How to Contact Us - Apple

You don't, that has never been a supported option. That is even true for Android phones where many who have tried with the various methods of flashing the Firmware have ended up with a bricked device. Not something an Apple user (or even Android user) would be happy with.