As of Business Manager 1.4.0 all add-ons are included by default, and don't need to be updated separately.
In the latest Zip file you download there is an
\---business-manager-[add-on_name]-[commit_hash] +---public +---readme.html \---updates incremental-update-x.x.x.zip
incremental-update-x.x.x.zip contains all updated files, so you do not have to worry about your current version. You can simply overwrite the files of your installation with the update files, it doesn't matter which version the Zip or your installation has.
incremental-update-x.x.x.zip file contains a
public directory. Extract the files in that directory to the root directory of your Business Manager installation.
When the updated files are uploaded, you have to log in with an Admin user and go to Settings > Update and click the "Run migrations" button.
The contents of the Zip file you download always has the latest version in the
public directory. You do not need to update if you install the add-on for the first time. Only if you have already installed the add-on and want to update to a newer version.