If you already have the Outlook Add-in installed and you want to update it, you need to remove it and reinstall it.
In Outlook on the Home ribbon select <Get Add-ins> button
A new window will open. Select the <My add-ins> menu and scroll to the Custom add-ins section
Use the <ellipsis> and select <Remove>
Then to reinstall it just follow our installation instructions in the article "Outlook Add-in - How to install"