We are getting this warning message whenever we're logging in to the Policy Manager:

The Currently installed SSL certificate for this gateway expires Wed, 27 Feb 2019

What does it mean and how to fix it?

The warning message indicates that the Gateway's default SSL certificate is expiring on the stated date.

Layer7 API Gateway 9.x

To replace the expiring certificate, please follow these instructions:

1) Create a new private key

2) If the certificate needs to be signed by a Certificate Authority (CA), generate a CSR and deliver it to the corresponding CA.  
   Once the certificate is signed by the CA, update the Gateway with this newly signed certificate.
  (Note: You can skip these steps if it is a self-signed certificate.)

3) Set the private key as the default ssl key

4) Restart the Gateway