I will answer my own question. I looked in the tutorial; I looked in the reference manual, but in each case I was led to the beginning of a score where you can retype the metronome length for a quarter note (♩ = 90), to begin the score. I searched the Internet, including this site, but found no answer for entering the metronome mark elsewhere in the score. Finally, through a detailed examination of the menus, bars, palette, etc., I found it. It's in the palette at the bottom of the manuscript window. In the palette, you click on the time signature, 4 over 4, and you get options including ♩ = ?. That's it! Click on "♩ = ?" and you get the metronome mark for a cursor, to click on where you want the time to change. Once it's in place, you can change the note (quarter, eighth, etc.) and/or numeral if you wish.
It took me hours, but now I can continue my work. I'm a fairly new owner of Notion, but I'm learning.