Tempo Marking Edits

I'm having trouble editing a tempo marking. I've tried several different methods to change the note (as described in the manual) with no success. I selected a note and used the Tempo palette and also tried the Alt-Shift-T approach. I tried editing it and using the Inspector. It's easy to change the default bpm from 80 to 126, but I cannot change the eighth note to a sixteenth note. My score should show that a sixteenth note equals 126. I'm not concerned with playback, just the engraving part. What am I missing? Thanks in advance!