You can do this with the metaflac
tool (man page), which should be available from the flac
package (I use debian).
You want to remove the old tag first, and then add the tag again with the correct values. Here is the artist tag for example:
metaflac --remove-tag=artist *.flac
metaflac --set-tag=artist="Beyoncé" *.flac
And here's a short demo of changing the artist tag on a couple of example files:
$ ls
11 - All Night.flac 12 - Formation.flac
$ metaflac --show-tag=artist *.flac
11 - All Night.flac:ARTIST=Beyonce
12 - Formation.flac:ARTIST=Beyonce
$ metaflac --remove-tag=artist *.flac
$ metaflac --show-tag=artist *.flac
$ metaflac --set-tag=artist="Beyoncé" *.flac
$ metaflac --show-tag=artist *.flac
11 - All Night.flac:artist=Beyoncé
12 - Formation.flac:artist=Beyoncé