In order to refresh the version in sites the add-in is installed proceed as follows:
You need to be a SharePoint Site Owner to perform the following steps.
1. Go to the site collection where you want to refresh the SharePoint Connector add-in.
2. Choose > SharePoint Connector for Confluence > context menu > About
Image 1: Site contents (click to enlarge)
3. In the details dialog there should be stated that a new version is available. Click GET IT.
Image 2: Details dialog if newer version is available (click to enlarge)
4. In the next dialog click Trust it.
Image 3: Trust dialog (click to enlarge)
If the "GET IT" button won't be provided in the details dialog no newer version is available for the SharePoint Online Connector add-in. In this case check the version displayed in the app catalog and compare with the version displayed in the details dialog for the installed version in the site.
Image 4: Details dialog if no newer version is available (click to enlarge)