It is most common to resend messages with the Failed status, for example if the message failed to send originally, likely due to a problem with your phone which you have now resolved.
It is most common to cancel messages with the Queued or Retrying status, for example if you realized after sending the messages that the recipients or content of your message were incorrect. (Note that it is not possible to cancel a message after your phone has sent it, even if it hasn't been delivered yet.)
On the Messages page, find the messages you want to resend or cancel via the Status tab (or any other tab except the Conversations tab). Then select the messages you want to resend or cancel using the checkbox next to each message, or the "select all" checkbox at the top of the page:
To select all messages matching the current filter even if there is more than one page of results, click Select all XXX messages.
Then, click Resend or Actions > Cancel sending messages.
If you have multiple phones, the Resend button will normally resend the message via the same phone that originally sent with. If you are using message routes, the Resend button will resend the message via the same route, which may send the message via a different phone. If there is a problem with one of your phones, this lets you easily reroute failed messages through a different phone after editing your route.