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.
Select Reply or Reply All (located at the bottom; to the right of 'Back').
Enter a message then select Send (located in the upper-right).