Do I turn off my phone when it is preparing for an update ?

You most certainly should not turn your iPhone off when it is updating, or downloading, or otherwise preparing for an update, Uncleham.

It should be on, be charged over 50% (or something like 20% if plugged in), not in Low Power mode (battery indicator shows yellow).

You iPhone must be connected to a good, reliable Wi-Fi.

It’s usually best to have it plugged in: so its charge will not drop below 50%, and it will stay connected to WiFi.

It should be OK for your iPhone to go to “sleep”, though, such as laying it face down. (Note: Sometimes, when an iPhone goes to “sleep” it may disconnect from Wi-Fi, unless it is plugged in.)

If you turn off your iPhone then it will not complete the task of preparing for the update. When your iPhone is in the process of updating then you should do nothing. Let the update process complete. Do not turn the iPhone off. Please have a little bit of patience and the process will complete. Note that it will not only prepare for the update but also verify the update and then install the update. It will take a little bit of time.

Axel F.