is there API endpoint for updating the existing documents/folder in D2 using D2 REST API?

I am assuming you are looking for meta data update. Here is the Rest end point -

POST http://<ipaddress:port>/<applicationcontext>/repositories/<reponame>/objects/<objectid>

Request body : In the properties bag below, add key value pair for properties which need to be updated

{"properties":{"title":"test"}}