Updated iPhone SE today from IOS 13 to 14.3. Thanks to a lot of help a few months ago from Nathan, Alex, and others I got through this monster.
They told me about getting iTunes on my Windows PC. Great idea since iTunes has a phone backup to PC feature and allows me to use my wired Ethernet instead of WIFI.
Did iTunes backup to my iPhone SE (but to my surprise after download it did an automatic backup before installing). Next surprise – Took about half hour and used about 5.5 GB. I thought it was in an error loop while watching my Ethernet counter but did some Googling and saw that was the expected size of the 14.3. Also it kept asking me to install purchased products from phone first but I did that and still got message but clicking continue finally started download.
After download got error 4000 on install attempt. Googled that and it was suggested phone locked or iTunes out of date version (I was current). I remembered on Windows update you usually have to do restart so I did Settings/General/Reset. After reboot I went back to iTunes and tried another update (reluctantly as I did not want another 5.5 GB download but miraculously it picked up at the install.) Another half hour later it finished after a lot of progress bars.
Also had to redo all my settings such as display size, clock alarm, etc as reset wiped those out.
And I thought Windows 10 update was a monster!!