ERS, which is on the column shift manual gear selector switch, or console mounted shifter
You can switch between DRIVE and ERS mode at any vehicle speed. When the shift lever is in the DRIVE position, the transmission will operate automatically, shifting between all available gears. Tapping the shift lever to the left (-) (console shift) or the ERS (-) switch (column shift) will activate ERS mode, display the current gear in the instrument cluster, and maintain that gear as the top available gear. Once in ERS mode, tapping the shift lever to the left (-) or right (+) (console shift) or the ERS (-) or (+) switch (column shift) will change the top available gear.
..................................................................
If everything is working properly:
When you start up your truck & put it in Drive, you are in First Gear, as your wheels start rotating, the truck automatically goes to 2nd gear, etc.
Once you are in any gear above First Gear, you can move the ERS button to the - position, it should back down 1 gear position. Once you are up to speed in say 4th gear, you can press the + button & the transmission will shift to 5th gear, do it twice & it will shift to 6th.