17.5.1 instant issues with the update

Yes, YOU need to fix this by contacting Apple Support. Hundreds of millions of users have updated to 17.5.1 without any problems, so this is a local problem, not a universal problem.

Syncing will pause if the phone is in low power mode, or is under 50% charge.

And you should also do a forced restart:

• Press and quickly release VOLUME UP

     then

• Press and quickly release VOLUME DOWN

     then

• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky. If done correctly this will always work unless the phone has a hardware problem. 

K_doe Said:

"17.5.1 instant issues with the update: As soon as I was done the update a pop up came in saying 'syncing has paused'. My phone flickers and other quirky things going on. We need to fix this!"

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Seems to be in a Boot Loop:

Your iPhone can at times be stuck in a Boot Loop. This typically occurs when you go about installing an update, and it stalls while in the process (which seems to be the case here). Use my User Tip: How to Fix an iPhone in a Boot Loop - User Tip

Update Using a Computer:

Go Here:  Update your iPhone Using a Computer - User Tip