[{"_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}}}},{$unset:{"tomes.$[tome].archive_number":1}},{arrayFilters:[{"tome.archive_number":{$exists:true}}],multi:true})