iOS 26.3 update prepares and restarts

Try Force Restarting your iPhone .

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo (this might take longer than 10 seconds).

"iOS 26.3 update prepares and restarts: when i try to update to ios 26.3, it just prepares the update and restarts."

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Getting Out of a Boot Loop:

Sounds like you are stuck in a Boot Loop. This typically occurs when you go about installing an updates, and it stalls while in the process of doing so. Use my User Tip: How to Fix an iPhone in a Boot Loop - User Tip

That sounds perfectly normal. The iPhone may restart several times during the update process.