This is a common issue for the developers to access ec2 instances by username and password authentication.  It requires a few steps to do that.

First, login as root using the key pairs.
Second, create a user

sudo adduser username

Third, you need to grant root level permission to the new user

visudo

Add following line

username ALL=(ALL) ALL

Forth, edit /etc/ssh/sshd_config setting and then set PasswordAuthentication to yes. Open the vim or nano

sudo vim /etc/ssh/sshd_config

Find and set PasswordAuthentication to yes

PasswordAuthentication yes

Finally, restart the ssh daemon

For Ubuntu

sudo service ssh restart

For CentOS

sudo service sshd restart

Done, now you are able to login into your ec2 instance without the key pairs.  Happy Hacking 🙂