On Step 2 you locate this folder
Windows: “C:\Program Files\3CX Phone System\Bin\nginx\conf\instance1”
Linux: “/var/lib/3cxpbx/Bin/nginx/conf/Instance1”
You copy the files manually there as described.
On Windows, you already know how to do it I'm sure
If you are on Debian just use an SFTP client app (i.e WinSCP) and login to the PBX machine and locate the folder that way. Then copy the files using the SFTP client
I downloaded the certificate files form the registrar. I renamed the .crt to -crt.pem and the pem file to -key.pem as the exact file names. I replaced the files and the service gets stuck on Stopping. The service will not start unless I remove the new files and replace with the old files for the old cert. Am I misreading the instructions? Missing something?
Check with your provider for instructions on either downloading the certificates in the correct format or converting them. Simply renaming won't work as @Edi Kojsi has mentioned.