Hi,
You, are right. There is no button to change the frame rate of a project in iMovie 10.
iMovie sets the frame rate of a project from the very first clip that is placed into a newly created project. You can later delete or move that clip without causing the frame rate to change.
So, create a new project and place a clip with a known frame rate that you want as the very first clip in the project. That will set the project at that frame rate. Then go to your old project and do an Edit/Select All, Edit/Copy, of the media in the project and then to an Edit/Paste of that media into the newly created project. Now your old project that you copied over is set at the frame rate of the new project. You can delete the first clip if you want, and that will not change the frame rate.
-- Rich