To create a folder, find the "Add folder" button on the left sidebar of your account homepage , below My presentations.
You'll be able to name the folder and add a short description. In order to edit or delete the folder, just click on it and look for the Settings button, next to its name. Deleting a folder won't affect the documents contained in it, as they will still be accessible in your dashboard.
In order to move a deck into a folder, go find the deck among your presentations and just hover over it. There, click on the More button (ellipsis, three little dots... you get it 😉), on the top right corner of the deck's preview and go to Folders. Just select the folder you want to move the presentation into and hit Save.
To remove a presentation from a folder, access the folder and hover over the presentation. Again, click on More, go to Folders and just deselect the folder in which the presentation is currently stored.