From the Accounts tile, go to Actions > Mark Invoices as Sent:
You can use the filters on the left to search for specific unsent invoices.
Clicking 'Select All' will select all unsent invoices. Entering a comma-separated list of invoice numbers into the 'Details' field at the top, then clicking outside of the box, will select those invoice numbers only:
Clicking 'Mark Sent' at the top and accepting the warning message will mark the invoices as 'Sent' on the date specified in the top-right of the window.