[Geoff Bacon]

Use the filters to display all songs that contain Beatles in their meta data
Long Press one of the songs until the checkboxes show
Use the floating box dialog to select all the displayed songs
Edit
Specify the required text in the required field e.g. "The Beatles" and OK
Then remove the unwanted "Beatles" entry as desctibed by Mike

Personally, I would prefer to call it "Beatles" rather than "The Beatles".

Geoff