From the home screen, select BlackBerry Hub. Alternatively, swipe right until the BlackBerry Hub appears.
If necessary, select the filter located in the lower left (e.g. Hub, Priority 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).
Enter a message then select Send (located in the upper-right).