From the command line, replacing the filename as necessary (you will need the password for the certificate, if one is assigned):
$ openssl pkcs12 -in wildcard.filename.com_2022.pfx -out wildcard.filename.com_2022.cer -nodes
If the above does not generate a certificate for you along with the private key, try adding the -legacy flag, like this:
$ openssl pkcs12 -in wildcard.filename.com_2022.pfx -out wildcard.filename.com_2022.cer -nodes -legacy
I needed to do this to convert a .pfx SSL certificate for use with Pagely.
EDIT: If you’re doing this for Pagely and the original certificate is a .pfx, after converting it you will likely need to open the .cer file in TextEdit and grab the two different parts.
One starting with
-----BEGIN PRIVATE KEY----- and one starting with
-----BEGIN CERTIFICATE----- and paste them in the appropriate fields after you select Add Certificate > Import Existing Private Key & Certificate options in the Pagely dashboard.