Samsung GALAXY MEGA2
The latest firmware for Samsung GALAXY MEGA2 (SM-G750A) is G750AUCS3BOH5, released on May 22, 2017 for 1 region (CSC). Every official firmware build for SM-G750A, 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/G750AUCS3BOH5
Box codes of SM-G750A for different colors and markets. Firmware is chosen by the model code SM-G750A and your region (CSC), not by the SKU.

SM-G750H

SM-G766U

SM-G850X

SM-G730V

SM-G556B
You can find the firmware on the model: SM-G750A page.
Yes, all firmware files for SM-G750A are 100% official, original, and obtained directly from Samsung servers.
To flash SM-G750A, 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 SM-G750A.