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Dashboard > Settings > Parameters
Set or Add TEMPORARY_SELF_SIGNED_CERTIFICATE_GENERATED and give it value of 1

Windows:
- On the 3CX Server open CMD.
- Type in the command: “C:\Program Files\3CX Phone System\Bin\PBXWizard\PbxConfigTool.exe” -renew-certificates

Linux:
- Log in via SSH.
- Type: /usr/lib/3cxpbx/PbxConfigTool -renew-certificates

Restart nginx just to be sure and the certificate should have been renewed.
For Linux, to restart nginx type: service nginx restart

You can check the C:\ProgramData\3CX\Data\Logs\PbxConfigTool.log to see log of the action.