You can only delete a project from Fieldwire while on the web (this feature is not currently available on the mobile app). To do so, simply log in and find your project on the Projects dashboard. Hover your cursor over the project's title, and you'll see a gear icon appear to the right of the title. Click on that and you'll see the delete option. Please note that only project Admins can delete projects.
If you are already within your project, you can delete it by going to the Settings section and clicking on the red "Delete" button in the top right corner.
Please also note that deleting a project is a permanent measure, and you will not be able to undo or recover any project data once you have done so. You may want to archive your project instead, as this will remove the project from your dashboard, yet you'll still be able to access it later if you wish to.