I just googled V40 LGUP and got this thread on XDA:
https://forum.xda-developers.com/t/guide-lgup-for-all-no-root-needed.3967858/
I haven't read the thread, but it would appear that there is an LGUP version for V40.
LGUP is the software we use with V30, G7, G8 and other models to cross flash or update/rollback LG phones. It's what carriers and service centers use. It does NOT require root. But it does require a version of LGUP for each LG model (a DLL file unique to each model). And again it appears we have it for V40.
You can generally flash any firmware version as long as LG or the carrier releases a KDZ file containing that firmware. These are encrypted files with the authentic firmware. AT&T and Sprint do not release KDZ files, but TMO and VZ do. Following are the KDZs for V405TA uploaded to lg-firmwares.com.
https://forum.xda-developers.com/t/guide-lgup-for-all-no-root-needed.3967858/
LGUP is generally a solid tool, and not difficult to use at all. But it DOES have features to completely wipe the phone (including serial/IMEI and other critical info) from which one should simply stay away!
You should do some reading on XDA to familiarize yourself before flashing away