How do I find a 4 digit passcode to install iPhone update

It’s the passcode that unlocks your phone. If you don’t remember it, you can’t update. You can erase the device and then enter a new passcode.

If you forgot your iPhone passcode - Apple Support

You can restore a backup after you erase the phone, but if you don’t have a backup, you have to start all over again. You may choose to forgo the update because you may not want to start all over again, but at some point, you may need to know the passcode.

It's really old fashioned to write down important passwords and passcodes and store them in a safe place, but also very smart.

We see dozens of posts each day from users who have forgotten their passcode and now must erase their phone to reset it and get going again. Too many of these same users never made a backup of their phones, so they lose everything when they "forget" their passcode.