Only the owner of a custom dashboard can delete it. If you're not the owner, you'll need to ask them to either unshare it or delete it for you.
Steps to Delete a Custom Dashboard
Verify Ownership
Make sure you're the owner of the dashboard you wish to delete. Only owners can perform this action.Open the Dashboard Menu
Click the three-dot menu icon located in the upper-right corner of the dashboard.Delete the Dashboard
Select Delete from the menu options. This will permanently remove the dashboard.