My desktop resolution is set to 1440x900 (75Hz) but Overwatch is only displaying 1440x900(60*) in-game. Is there a way to “unlock” 75Hz in game?
Depends, if you are using a 1.1 or 1.2 HDMI cable you may be limited to only 60 FPS. There are also settings in video options to increase FPS. I have full tips available here to help you choose the best possible settings for your system: