You can access the Preferences by going to Edit > Preferences on Windows or Lightroom > Preferences on Mac. Once the Preferences dialog box appears click the External Editing tab to access these settings.

By default Lightroom always chooses the most current version of Photoshop you have installed as the primary external editor, but you can configure other applications to be used in the Additional External Editor section.

The File Format, Color Space and Bit Depth options are the same as the previous version of Lightroom, and choosing the default settings is recommended. The external editor video tutorial Matt Kloskowski created for Lightroom 1 is still valid in that regard if you want to learn more.

Read  Configuring External Editors over at peachpit.com.