Prerequisite:
You should have the certificate (.pfx).
For Workspot Daas certificate: Needs the daas20 certificate.
For Customer-owned Certificates: Need their certificates.
You need the certificate password.
Ensure the Gateway is in maintenance and has no user connections.
Steps to upgrade the certificate manually:
Log in to the Gateway and copy the files into the Gateway server.
Open the Remote Desktop Gateway Manager on the Gateway Server.

Right-click on the Gateway server and select properties.

Click on the SSL Certificate tab and select the “Import a certificate into the RD Gateway “Gateway_name” Certificates (Local Computer)/Personal store” option.

Click on the Browse and Import Certificate…

Upload the copied certificate. You will be prompted to enter the Private Key Password.

Click on Okay to finish. You can see the new certification Expiration date below:

You can also verify in the Watch; it may take a couple of minutes to update the changes. You can restart the Workspot Gateway Agent service to replicate it immediately.
Please Note: If a customer has a firewall or network appliance between the Clients and the gateways, it is configured for SSL inspection; ensure the certificate is also updated on the network appliance.
Note: Make sure to delete the certificate and password, in case you saved in the gateway or any other location.