The following instructions work in my 2019 Ascent Limited (Chapter 3-10 of the owner's manual). They should also work for you:
Step 1- Make sure the Ignition Switch is in the "ON" position (the engine doesn't have to be running, but the dash should be lit up)
Step 2 - Press the physical Home button below the LCD panel. In the panel, touch Settings, then Vehicle, and finally, Clock Adjustment. Select Manual.
Note: This is the critical step, otherwise you can't get into the Time/Date settings in the Multi-function display (MFD)
Step 3 - Now, use the physical buttons above the LCD panel. Press the down arrow to cycle through the various screens in the MFD until you get get to Settings (the bottom most "bread crumb" indicator on the left side of the MFD will be lit). Press and hold the View or Enter button to open the MFD settings menu.
Step 4 - The Time/Date entry should now be highlighted. Press the View or Enter button.
Step 5 - Use the down arrow button to cycle through the options in the Time/Date panel. Stop on 12H and press the View or Enter button. Press the down arrow button to change it to show 24H. Press the View or Enter button again to return to the Time/Date panel. Press the down arrow button until you get to Set. Press the View or Enter button to make the change permanent. This will also return you to the previous menu.
Step 6 - Using the up arrow button, select Go Back and press the View or Enter button. Do this again to get out of the Settings screen.
Step 7 - In the LCD panel, you should still be in Vehicle settings, Clock Adjustment. Set it for Automatic, then touch Go Back. Press the physical Home button to return to the main display.