Here's some basic steps to try to get the update downloading again (via iGleeksBlog). 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.
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
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.