To make apps load faster, programs utilize a "cache" which stores a copy of the program (or add-in) on the users computer to access much faster on subsequent opens.

This however can cause version problems as Excel will serve outdated copies of the add-in.
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The best way to make sure you have the most recent version is to clear the Office cache.
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If changes you've made do not seem to take effect, try clearing the Office cache on your computer. 

Delete the content of the folder %LOCALAPPDATA%\Microsoft\Office\16.0\Wef\.

Delete the content of the folder /Users/{your_name_on_the_device}/Library/Containers/com.Microsoft.OsfWebHost/Data/.