This is probably not the most efficient, but you can try:
UPDATE default d SET _id = (SELECT RAW x.num FROM (SELECT ROW_NUMBER() OVER() AS num, meta(d1).id AS docId FROM default d1) x WHERE x.docId = meta(d).id)[0];
You can modify the innermost subquery to order the documents the way you like.
Requires Couchbase Server version 6.5 or above.
MERGE INTO default AS m USING (SELECT META(t).id, ROW_NUMBER() OVER() AS num FROM default AS t) AS s
ON KEY s.id
WHEN MATCHED THEN UPDATE SET m._id = s.num;