Got it working. Thanks, everyone!
Must make at least 8 GB free space on Windows O/S "C:" drive to extract the 5.6 GB download.
The latest update to iPhone to date is iOS 15.2. In its entirety (not the incremental update) this file, which is named "iPhone_4.7_P3_15.2_19C56_Restore.ipsw" and resides on appldnld.apple.com, is about 5.6GB. Yes, larger than a DVD movie! It is suggested to download this file on a PC that is connected by ethernet wire LAN, not WiFi; then the download will be faster. On a LAN at DSL (3 GB/s) speeds, the download takes about 5 hours. Over Wifi, at same DSL routed speeds, it's about a day and a half (wireless is much slower and much less reliable).
To transfer this huge file to the iTunes PC, such as a MS Surface 3, which is wireless only and has no ethernet port, you will have to use a USB Thumb Drive that is large enough. However, you will first need to (re)format your USB Thumb Drive for "exFAT" filesystem; the default FAT32 will not hold this huge .ipsw update file, since 4GB is the maximum size for FAT32.
Copy/paste the ipsw file to the D: drive of the MS Surface 3. You cannot install it directly from the USB Thumb Drive, because the MS Surface 3 only has one USB port, and you need that for the target iPhone being updated. Further, DO NOT copy/paste the ipsw file to the C: drive, because iTunes extracts only to the C: drive (can't be changed) and you need all the space you can get.
In iTunes, remember that ...
... "Update" means no data loss (re-load O/S only), and
... "Recover" means total data loss (wipe and factory restore).
Press and Hold the SHIFT key while clicking the Update button. This will present a file dialog box and allow you to choose the ipsw file that you downloaded earlier.
From then on, the process should proceed smoothly. When all is said and done, don't forget to delete the huge ipsw file from D: drive; it is probably still on the USB Thumb Drive, unless you already removed it from there, too.
You've probably never seen this screen, but if you do, it's a very good sign that iPhone will come back and nothing will be missing.
Oops! If you see this screen, it means you must make more space on Windows system drive C:, deleting or moving as much as possible (namely, the ipsw file) to other drives.
Remember to hold SHIFT key while clicking Update -- otherwise iTunes will download the huge file, again! Also, remember that "Update" means no data loss, and "Restore" means total data loss and complete wipe of i-device.