To view and edit the metadata about your score, select Score Information from the File menu.
In addition to displaying the score title, subtitle, composer and lyricist, the Score Information window lets you view and edit extra information for cataloguing or indexing the score file.
Editing the Title, Subtitle, Composer and Lyricist
Another way to edit the title, subtitle, composer and lyricist fields is to click the related field on the page when the Floating Text tool is selected.
To make the hidden fields visible, select Layout from the Score menu.
Please check the post How to Show and Hide the Score Header in Mus2 for a detailed explanation.