I figure it out like this
var myFolder = Folder.selectDialog("Select Indesign Folder");
var myIndsnFiles = myFolder.getFiles("*.indd");
for(k=0; k<myIndsnFiles.length; k++)
{
app.open(myIndsnFiles[k]);
app.activeDocument.transparencyPreferences.blendingSpace = BlendingSpace.RGB;
app.activeDocument.save();
app.activeDocument.close();
}