From your Analytics tab, simply hover over the matrix that you no longer need and select the three dots at the top of the matrix. Select delete and in the pop up menu, confirm the deletion of that matrix.
From your Analytics tab, hover over the project that you wish to delete and select the bin icon that appears:
⚠️ Confirming the deletion of an Analytics project will delete all the matrices within the project. This action cannot be undone so only use this option when you are certain that you wish to do so.