Success.
I downloaded the 845,837 byte file ("RUU_JEWEL_CL_JB43_SENSE50_MR_Sprint_WWE_4.13.651.4_Radio_1.13.11.1105_NV_3.02_003_PRL25007_release_353492_signed_one_step.exe") and updated my software from 3.17.651.9 to 4.13.651.4.
Unfortunately, it wouldn't work on my Win 10 PC, or my other Win10 PC, or my Win10 laptop because the USB stopped working with a "device failed enumeration" error causing Win10 to disable the USB. I tried in vain on my Win 7 PC, but it finally worked on my XP laptop.
Now, I've got 4.3 (from 4.1.1) on my HTC EVO 4G LTE, with HTC Sense version 5.0. It's a victory, however small since I don't even know what differences there are. It was a factory reset, and I have reloaded the apps I want, reset my wallpaper and display preferences.
Phil in gorgeous Young Harris, Georgia, USA.