This is not quite trivial in general case, since it varies depending on title implementation by specific theme.
The historical way has been wp_title() call in theme’s template. The newer method is hook–based Title Tag.
So it is hard to say specifically without knowing/seeing code of a specific theme.
This is one of the reasons for high popularity of SEO plugins, for many of which manipulating titles in more user–centric ways is one of the main features.