Click the "Delete Existing Stocks" button on the portfolio overview tab.
Delete 1 Stock/Fund
Select a stock from your portfolio to delete. This will delete the entire entry of that selected stock/fund from your portfolio.
Delete All
Click the "Delete Everything" sub tab. You can also check a box to "Permanently Remove All Holdings From Your Portfolio". This will make your portfolio empty, deleting all entries.
Click "Update" to delete the selection from portfolio.