Start by logging into SquirrelMail.
If you want to increase the number of messages you can select at a time, you have two options:
Under Options > Display Preferences you can increase the number of messages show per page.
Or, you can click the "Show All" link near the bottom of the screen. Please note, depending on the number of messages in your Inbox, this could take several minutes for all of the messages to display or may time out.
Click the "Select All" link near the bottom of the screen.
Click the "Delete" button to move the messages to the "Trash" folder. Depending on the number of messages being moved, this can take several minutes.
Once you have moved all of the messages, click the "Purge" link beside "Trash" to finish deleting the messages. This may take several minutes, depending on how many messages are being deleted.