While EnergyPlus plus supports most curve types for Normalized Boiler Efficiency, and the OpenStudio API method to create a Boiler accepts any type of curve, the OpenStudio Application currently doesn't let you change the curve type for an existing Boiler.

The boilers in the default library for the OpenStudio application uses Biquadratic, and you have found one that uses Cubic. Here are two possible approaches to use a different curve type.

1. Find another OSM that uses your desired curve type, and load the OSM as a library and then drag in the Boiler, and modify settings as needed.
2. Write a measure to change the curve type for existing boilers. Code would like something like what is below. Then you would could modify the curve values in the GUI, or make them measure arguments.

model.getBoilerHotWaters.each do |boiler|
  curve = OpenStudio::Model::CurveQuadratic.new(model)
  boiler.setNormalizedBoilerEfficiencyCurve(curve)
end