My client had installed AWS Marketplace Bitnami WordPress and he do not have any .pem file & credentials associated with that EC2 instance. We need to change something in wp-config.php & .htaccess file. Now we are not able to do this.
I googled but not found anything fruitful.
Sorry, I am posting an answer to my own question. As after 1-week discussion with AWS support they share an option to me where I can use my client's EC2 instance (that is not associated with any keypair/.pem file). They reset my setting and shared lines of code that I need to add in textarea under "View/Change User Data".
And this lines of code had user:password string. With those credentials, I connected to SSH and completed my job... :)
Sorry for security reason I can not share the lines of code. But I answered my because I am sure this answer will help someone in future. Actually needy will get a hint from my answer (i.e. "View/Change User Data") and he/she can directly contact AWS Support.