I'm browsing the SSL certificate list which I got from Comodo, and there are different types of Contents with the below mentioned extentions. I have experience in uploading certificates to F5 which is in .pfx format that contains both certificate and key in a single file but the newly received CA certificates giving me a hard time understanding which one is a cert file and which one is key file. Can someone help.
.cer, crt, and .p7b files are almost always (public) certificates. A private key would normally have a .key or .pem extension, but of course can be stored in pkcs12 .p12 or .pfx file. It doesn't look like any of the certificates you've listed are private keys, but you may still need to open them all in a text editor to know for sure.
If the file contains a string that looks like this
-----BEGIN RSA PRIVATE KEY-----
then that will be a private key.
If you are renewing a certificate then it will simply re-use the already existing key. The only thing that gets renewed is the public certificate. Have you tried following this guide?
Importing a renewed SSL certificate
When you import a renewed SSL certificate, you overwrite the existing certificate/key with the one you are importing. The SSL profile then automatically uses the renewed certificate to encrypt the SSL sessions.
Important: Existing connections continue to use the old SSL certificate until the connection completes or are renegotiated or until TMM is restarted.
Impact of procedure: Performing the following procedure should not have any impact to the existing traffic and new traffic will utilize the new certificate.
Note: For BIG-IP 12.x and earlier, navigate to System > File Management > SSL Certificate List.
To be honest I always generate a completely new PFX or CSR in order to renew both the certificate and key for security reasons. It will also give me the possibility to revert back to the old certificate in case there are some issues with the newly generated certificate/key. The only thing I have to do after uploading the new certificate/key pair is make the switch in the Client SSL Profile.
Here's some SSL related content from Technical Articles that might pique your interest: