Use this online SSL Converter to convert SSL certificates to and from different formats such as pem, der, p7b, and pfx.

Step 1: Open https://www.sslshopper.com/ssl-converter.html
 

Step 2: Browse and select your certificate.crt file.

Step 3: Then select Type To convert To PFX/PKCS#12

Step 3: Then browse and select the private.key file.

Step 4: Set a password for the PFX file and remember that this password will be required when you install the PFX file in your Windows server.

Step 5: Now click on Convert Certificate and save the PFX file.