Backup the Synchronizer database - If you have a default installation, you should be able to find the config.sdf file in this path: C:\inetpub\wwwroot\tfs-jira-synchronizer\App_Data\config.sdf - In the machine where Synchronizer is installed;
This file contains the Sync Profiles settings;
Go to IIS Manager → Application Pools → Stop the TFS4JIRA app pool
Open Control Panel --> Program and Features --> Search for TFS4JIRA --> Select it and uninstall it;
You can download the version 7.8.6 here: Synchronizer v7.8.6;
Execute the installer for version 7.8.6 - run the installer as administrator;
Let the installer complete the installation (finish and close);
Once the installer is finished, the Synchronizer will start run automatically;
Access the Synchronizer to see if everything is alright after logging in.