I'm assuming you mean setting the frame rate for your recording. I don't think it can be changed on an existing movie, but I might be wrong about that.
For recording, after you open the recording dialogue, you should see item #3 "Recording quality:"
Select the down-arrow to the right of the first window and from the pull-down menu, choose "...user defined" or "...user specific" (depends on the file format you want).
The simplist to discuss is "MXV: User defined..."
Once you have that chosen, click "configuration"
In the configuration editor, you will see "Frame rate:" and one of the options is 15.00
Note: I found that information after selecting "Video recording" as the recording source. I don't have my camcorder connected right now, but hopefully, the same instructions work with the other recording sources, too.