I have a variable BOX at the configuration A>B.
I want to update the value of this variable BOX to some other value using rest api .How can I do that.
You may create a separate configuration in QB to execute groovy script to update variable:
groovy:
def conf = system.configurationManager.get("path/to/conf");
conf.getVar("varName").setValue(vars.getValue("varName"));
system.configurationManager.save(conf);
Then trigger build of this configuration via RESTful API and pass in varName with desired value:
This way you can virtually change anything you want via RESTful API.