This is the official AT&T LG K40 User manual revision 1.1 in English and Spanish provided from the manufacture.
In order to reflect the changes in the software update, LG official recently revised the AT&T LG K40 LM-X320AS User manual and made it available for download.
For your information, the model name for AT&T Branded LG K40 is LMX420AS (LM-X420AS, X420AS). The AT&T prepaid LG Xpression Plus 2 is a variant of the LG K40.
Download AT&T LG K40 User Manual / Guide
As of today (March 23, 2020), The latest software update for AT&T LG K40 LM-X320AS is software version X420AS10h which includes the March 2020 Android security Patch.
AT&T LG K40 comes with Android 9.0 Pie OS out of the BOX. We are not sure if LG will offer the Android 10 OS Update for the AT&T LG K40.
Like many other devices from AT&T, The LG K40 with model number LMX320AS (LM-X320AS, X320AS) supports firmware update via OTA only.
It does not support update through LG Bridge.
There is no way to download the stock kdz firmware for LG Xpression Plus 2 either.