From the home screen, swipe right until the BlackBerry Hub appears.
If necessary, select the filter located in the lower left (e.g. Hub, Notifications, BBM, Text Messages, Email account, Calls, Emergency Alerts) then select the appropriate email account (e.g. Gmail, Yahoo!, Live, etc.).
Select a message. If the message contains an attachment, select the attachment to download and open it.
Select Reply/Reply All (located at the bottom).
Enter a message then select Send (located in the upper-right).