You can change the status of a request, and the message you provide, very easily.
To change the status, simply navigate to the relevant request. You will see the "Request status" panel in the top-right corner.
If you haven't already updated the status, then it will currently be set at "Not Reviewed". Click on the arrow next to the current status and you'll be presented with a drop-down choice of statuses.
Select the status you want and you'll be given the option to choose between a saved message, or to write a custom message of your own.
You also have a couple of checkboxes to consider.
If you customize the response, you can check "Save response for future use" in order to add this new response to your saved responses.
You can check the "Email your response..." box to automatically sent an email notification to everyone subscribed to the request.
The JIRA box is optional depending on whether you've installed the JIRA add-on.
Once you're happy with the status and the message, click the button at the bottom of the pop-up to confirm the change.