The configuration of your Ardor and Nxt nodes is configured using properties set in the nxt.properties file.
The nxt.properties file is a text file which can be created using any text editor (notepad on Windows and vi on Linux and Mac will do)
Create or update the nxt.properties file in the conf directory under the user directory
For example, to open API access to your node from remote client, add the following lines to nxt.properties:
Every change to properties will only take effect after you restart your node.
You can use the nxt-default.properties file from the conf directory of the Ardor or Nxt installation as reference to see the existing properties and their documentation.
Never modify the nxt-default.properties file since your modifications will be overwritten by any upgrade to a new version.
Another configuration file you should never modify is nxt-installer.properties under the user directory conf directory. This file stores the configuration selected during the installation.
Any setting set in nxt.properties will override settings in nxt-default.properties and nxt-installer.properties
For advanced users:
The location from which the property files are loaded can be customized using a Java command line argument named according to the property file name.
java ... -Dnxt.properties=./conf/examples/testnet.properties ...
Will tell the Ardor or Nxt node to load the nxt.properties file from the ./conf/examples/testnet.properties folder.