It is quite easy.

Highlight the measure that you want to begin the slow-down.
Hit the F5 key (or choose "Bar Settings..." from the menus).
You can change the tempo to a specific (slower) tempo....
Or choose to change by a percentage (like 85%).

Repeat that in the next measure - with a slower speed...

If you want to get more "granular", you can change all of your final measures to 2 beat measures (just above the tempo change) and change it more gradually (adding more measures to accommodate that...


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