1. first make sure filezilla is installed
sudo apt-get install filezilla
2. open up filezilla and navigate to Edit->Settings menu and select SFTP option there. Click on the Add key File... button, then select your .pem key file. It will ask to convert it to .ppk, do so.. then click on OK.
3. Now, on the main window, enter the following information.
Host : Your Server's Ip or Your Public DNS
Username : ubuntu
like shown below in the image
then click on Quickconnect.
Now wait a few seconds... Done!