Here's some basic steps to try to get the update downloading again (via [iGleeksBlog](https://www.igeeksblog.com/ios-update-stuck-at-update-requested/)).  You didn't mention what you've already tried so I'm assuming you didn't try any of these yet:

> #1. Network Reset
> 
> One of the first things you should make sure of is a stable network.
> If you’re on cellular or spotty Wi-fi, there can be problems when your
> iPhone tries to contact Apple’s servers to download the update.
> 
> Once you are stuck at “Update Requested…”, it’s best to turn off the
> networks on your iPhone and reconnect it. If you are on a 3G or LTE
> data network, try to switch to a stable Wi-Fi service before updating.
> 
> #2. Hard Reset
> 
> If network is not the issue, you can try doing a hard-reset to get the
> update working again. To hard-reset your iPhone/iPad:
> 
> Press and hold the Power and Home button till the screen goes black
> Now, press the Power button to boot the iPhone/iPad After you
> hard-reset, check the Software Update page again. Either the “Update
> requested” changes to downloading or you get to start all over again
> to download and install the update.
> 
> #3. iTunes
> 
> If you see the “Update requested” for hours after you’ve initiated the
> update, and all the above methods haven’t fixed it, there’s probably
> only one way of getting the update done – via iTunes. Surely, this
> beats the very purpose of an OTA (simplicity) but given that the
> iPhone is stuck in the update, you might want to switch to the old
> method.
> 
> Connect your iPhone to the PC/Mac, fire up iTunes and from the Summary
> tab, click on Update. In most cases, iTunes will automatically show a
> pop-up that says there’s an update for your iPhone/iPad.