At the top of the extension menu you can see the version (so you know your new install worked). If install did not work, first go to preferences, add-ons, open the Migopos Toolbox, go to Open install folder. There remove the old script file (__init__.py) and replace it with the new from the zip. You may also want to remove the content of the __pycache__ folder.

Documentation has a tutorial on how to install and enable the extension.