
Samsung Galaxy Prevail 2 4GB (Virgin Mobile)
The latest firmware for Samsung Galaxy Prevail 2 4GB (Virgin Mobile) (SPH-M840) is M840VPBOI1 with Android 4.1.2, released on September 13, 2015 for 3 regions (CSC). Every official firmware build for SPH-M840, newest first: one row per build with all regions (CSC) that received it.
Each firmware build has its own page at https://samfirm.shop/build/{PDA}, where {PDA} is the build number from the list above. Example: https://samfirm.shop/build/M840VPBOI1

SPH-L900

SPH-L710T

SPH-L720T

SPH-L600

SPH-L520
You can find the firmware on the model: SPH-M840 page.
Yes, all firmware files for SPH-M840 are 100% official, original, and obtained directly from Samsung servers.
To flash SPH-M840, you need to download the appropriate firmware, put your device into Download Mode, and use the Odin tool to load the BL, AP, CP, and CSC files.
Downgrading is only possible if the bootloader version (binary bit) of the old firmware matches the current one. Always check the binary version before flashing SPH-M840.