There are two possible ways to update expired certificate:
1.Rollback VMAS and Server time to the moment when certificate was valid and perform PowerShell actions to update cert.
2.Reinstall Workflow Manager
References:
https://sharepoint.stackexchange.com/questions/247107/cannot-update-expired-servicebus-certificate
http://www.harbar.net/articles/wfm3.aspx
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