To install the Duplicator the first thing you need to do is download the plugin, you can easily do that here: https://wordpress.org/plugins/duplicator/installation/
When have finished downloading the plugin , you need to go to your admin page in wordpress, and then click plugins.
Then you click on Add new and when you are inside click on Upload plugin.
When you click on the upload plugin it will take you to a page where you can click choose file, and locate the zip file on your computer.
This is how it'll look like when uploading the plugin, where you can also activate the plugin directly.
And when this is active you will get this in the menu to the left and then you can continue with this guide.