How to force signature synchronisation in the Add-in?

In Outlook, go to the "Message" menu, then click the logo with the four squares and follow the red arrow below: 

-> Access the Boost My Mail application by clicking it: 

You can then force signature synchronisation using the "Refresh" button at the very bottom of the page. Once synchronisation is complete, a small green logo appears, confirming that the synchronisation was successfully completed, along with the date and time.