Changing titles in Uploader for “Posts”

you have the “media_view_strings” filter for this :

add_filter("media_view_strings", function ($strings, $post) {
    $strings["insertMediaTitle"] = "Upload Content";
    $strings["uploadFilesTitle"] = "Upload";
    $strings["mediaLibraryTitle"] = "Your Content";

    return $strings;
}, 10, 2);