How do I revert to a previous iOS version?

If you just updated to iOS 26, you just lost luck. Apple deleted the Restore Factory iOS version button. The only way to fix it is to buy a phone earlier than iPhone 17 / IPhone Air. It is a big risk to degrade for security reasons.

No.

Nor should you want to. It includes not only feature upgrades, but also defect repairs, security enhancements, etc.

If you tell us your particular issue, perhaps we can assist.