Assuming you have VLC (http://www.videolan.org/vlc/) or Media Player Classic (http://mpc-hc.sourceforge.net/) installed.
You should be able to drag the subtitles file into the media player while the movie (anime?) is going.
If you mean there are subtitles already in the movie, but you can't see them, in VLC click Video > Subtitles Track > English, or in Media Player Classic click Navigate > Subtitle Language > English.