Can now do in the interface:
Click on Edit from the main menu
Choose Editor Preferences
Under Level Editor section in left menu choose Viewports
Check box that says Invert Mouse Look Y Axis
Click Set as Default button
But if that doesn’t work you can try:
Go to \Program Files\Epic Games\UE_4.24\Engine\Config
Edit file BaseGame.ini
Under section [/Script/Engine.PlayerController]
Change InputPitchScale= to a positive number
Restart the Unreal Editor