Navigate to the document that you want to edit the title for and click the 'Show Menu' button as shown below.
Click 'Edit document title' from the list shown.
Write your new title in the box shown below and click ok.
The title of this document has now been edited for both you, your team and customers.
Note: This changes the title for this document only.
If you wish to change the default title of all new documents within a board, please consult our article: How to Change the Default Title of All New Documents