Hi I hope I am able to answer your question effectively.
In QB, across the top, where you see "File, Edit, View, etc" click on "Lists" Then "Class Lists" A list of your classes will appear. At the bottom of this page there is a button that says "Class" click that and a drop down will give you options to create a new class, delete a class, edit a class, etc.
If nothing has been coded to the "default" that QB created on its own, then you can simply delete it. If something has been entered using that class, you can try merging the two classes, to do this you click "edit" and rename the first class the exact same name as the one you want to use, Qb will generally pop up a message that says this class already exists, do you want to merge them, if you get that message simply click yes and your problem should be solved.
Some people have said they don't get this message, that they have to manually change everything coded to the incorrect class to the correct one then delete the incorrect class.
Either way this should solve your issue with running the report. Once you have the report set up the way you like it, you can click memorize so you don't have to do it everytime.