===MAKE SURE TO CLOSE THE GAME FIRST===

1. Go to your steam folder steam/steamapps/common/ringofelysium/userdata/
2. Find and edit the file EuSystemSettingsConfigCurrent.xml using notepad or notepad++
3. Change the value "iSensitivityRatioVertical" to negative like 60 to -60. (Set it to whatever number you're comfortable playing at.)
4. Save the file and restart the game.

***If you change any binds in game the game will automatically set the value for "iSensitivityRatioVertical" 0 so you will not be able to move your mouse up or down at all. At that point you need to quit the game change the value in EuSystemSettingsConfigCurrent.xml again and save the file.