How to promote hidden mongo node to primary?
We are trying to implement DR setup for our mongodb replica set and
  currently three nodes in DR are configured to run as hidden nodes.


As per MongoDB documentation Configure a Hidden Replica Set Member Hidden members are part of a replica set but cannot become primary and are invisible to client applications. Hidden members may vote in elections.

The most common use of hidden nodes is to support delayed members. If you only need to prevent a member from becoming primary, configure a priority 0 member.

Member Configuration Document

To configure a secondary member as hidden, set its members[n].priority value to 0 and set its members[n].hidden value to true in its member configuration:

{
  "_id" : &lt;num&gt;
  "host" : &lt;hostname:port&gt;,
  "priority" : 0,
  "hidden" : true
}


Configuration Procedure

The following example hides the secondary member currently at the index 0 in the members array. To configure a hidden member, use the following sequence of operations in a mongo shell connected to the primary, specifying the member to configure by its array index in the members array:

cfg = rs.conf()
cfg.members[0].priority = 0
cfg.members[0].hidden = true
rs.reconfig(cfg)


After re-configuring the set, this secondary member has a priority of 0 so that it cannot become primary and is hidden. The other members in the set will not advertise the hidden member in the isMaster or db.isMaster() output.


  We are trying to test failover to DR which will require promoting
  hidden node to primary and secondary respectively. What is the best
  way to achieve this?


Note with WARNING:


The rs.reconfig()
shell method can force the current primary to step down, which causes
an election.
When the primary steps down, the mongod
closes all client connections. While this typically takes 10-20
seconds, try to make these changes during scheduled maintenance
periods.
Avoid reconfiguring replica sets that contain members of different
MongoDB versions as validation rules may differ across MongoDB
versions.