OpenSSL is used for many things other than running encryption on a website. It is also used for the generation of CSR keypairs, and more importantly within this article converting.
The Italic parts in the conversions below are examples of you own files, or your own unique naming conventions adapt these Italic name examples to your own files names for openssl commands.
Note: .pem, .cer, crt. are all the same type of x509/pem certificate only with different extensions.