If you want to delete a document, go in your account to Archive. Then select the recycle bin icon to the right of the document that you want to delete.
In the app, you can swipe to the left in the overview of documents or click on the three dots in the top right corner of the app in an open document.
Before deleting a document, bear in mind that this is permanent and that you will no longer be able to retrieve the document on Doccle. The archives of your Doccle account are free and unlimited, so it's not necessary to delete your documents.