[{"_id":1,"tomes":[{"number":1,"archive_number":1},{"number":2,"archive_number":1}]},{"_id":2,"tomes":[{"number":1,"archive_number":1},{"number":2,"archive_number":1},{"number":3,"archive_number":1}]}]

db.collection.update({"tomes":{$elemMatch:{"archive_number":{$exists:true}}}},{$unset:{"tomes.$[].archive_number":1}},{multi:true})