BREAKING: Multiplayer is NOT coming to MW2 Remastered. Instead, Activision plans to add "more classic map experiences  to Modern Warfare.

Firstly, open Steam and select MW2 from your games library. Then, click on Properties, followed by Local Files, and finally, click on the Verify Integrity of Game Files &  option. Steam will then automatically check for updates and update your game files to the most recent version available.