How do I undo an IOS upgrade?

Sophiencharlie wrote:

1. After upgrading to IOS 26.3 my phone

will no longer let me add appointments to my calendar

2. Siri no longer works

3. Can't use voice recorder for text message

Is there a way to undo upgrade?

Nope, you cannot undo an update. But then most people aren't having the issues you've listed after updating to iOS 26.3. At a minimum, Force Restart your iPhone (which I do immediately after EVERY iOS update) --> https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

Sophiencharlie wrote:

Is there a way to undo upgrade?

No. Apple has never supported downgrading production versions of iOS.

Restart your iPhone and test. A restart often clears glitches after an update.

https://support.apple.com/en-us/118259

There are almost no reports of the issues you describe by other users. There are now 1.5 billion Apple device users in the world. And if your issues were common these forums would be brought to a halt by people complaining.

Are you having a device storage issue? Why are you deleting apps? While pointing out that others have had success may not be specifically helpful, it does provide a framework for moving forward. What other troubleshooting steps have you performed in attempting to resolve the issue? Specifically what model device are you using?