To use the Multi Delete tool you will need to access your data from the Income or Expense tabs. Once there you have the option of simply clicking "Edit Multiple,"or refining your search using the search fields above the transactions. You can search by date, account, category, or name. Once you have chosen the desired fields click filter.
After the page loads you are ready to edit. Click at least one transaction, you'll see the option to delete those transactions at the bottom/right of the Edit Multiple box.