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 route 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 route has sent it, even if it hasn't been delivered yet.)
On the Messages page, find the messages you want to resend or cancel by selecting a Status filter as shown in the screenshot below (and add any other relevant filters such as Date or Type). 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 your project has multiple routes, the Resend button will let you choose which route to resend the message from. (If you are using custom routes, selecting "same route" will resend the message via the same custom route, which may send the message via a different basic route.) If there is a problem with one of your routes, this lets you easily reroute failed messages through a different route.