To open the Subtitle Editor, click on the speech bubble in the toolbar to open up the below window.

Indicate the primary language of the existing subtitles and use the option "OPEN an existing SRT/STL subtitle file".

If there are Subtitle Presets defined for your production, choose the one that you want to use. This will define the styling and timing rules for your subtitles. You can read more about the configurations in this article about Subtitle Presets). 

 Once you click on 'Open SRT/STL', a dialog will pop up by which you can select an existing file that contains subtitles.