Click on the gear icon below the star icon on a project to open the Project Properties panel. If you are inside a project, you can access its properties by clicking on the Settings button at the top right corner of the screen.
Click on the Project Name to edit it.
You may also click on the project to open and then click on the Project Name to edit.
Project admins and members can edit a public project's name.
Only the project admins can edit a private project's name.
Here's the link to the complete Workspace roles.