I already have purchased SSL certificate and i have received certificate and a .pem file as a private key? from the supplier; now i need to convert this .pem key into .key for b
CA's don't ask for your private keys! They only asks for CSR to issue a certificate for you.
If they have your private key, it's possible that your SSL certificate will be compromised and end up being revoked.
Your .key
file is generated during CSR generation and, most probably, it's somewhere on your PC where you generated the CSR.
That's why private key is called "Private" - because nobody can have that file except you.