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. 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 click on it. There, click on the More button (ellipsis) 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 click on the presentation. Again, click on More, go to Folders and just deselect the folder in which the presentation is currently stored.