You can use this operation to change the account properties of the primary site administrator.
Note:The primary site administrator account is stored within ArcGIS Server and can be used to log into Manager and perform administrative workflows. This operation must be invoked over an SSL connection as requires an exchange of vital credentials.
You can optionally provide a new name for the primary site administrator account.
The password for the new primary site administrator account.
The response format. The default response format is html.
Values: html | json| pjson
Below is a sample POST request for update:
POST /webadaptor/admin/security/psa/update HTTP/1.1 Host: machine.domain.com Content-Type: application/x-www-form-urlencoded Content-Length:  username=agsadmin&password=newSecret&f=json
JSON Response example