Replying to a message is very easy.  Click on the message to which you want to reply.  In the case below it has been decided to reply to the message from Katherine Lee

To reply, click on the Reply icon. To reply to all the people that received the message, click on Reply All.

The message box opens with her address already there, although it may be 'underneath'.  Write your message in the area indicated.  It is wise to delete any part of his message which is not required in the reply.  When you have finished the reply, just click Send and it will go back to her. 

A note on e-mail netiquette.  Do not correct grammar or spelling mistakes in other people's messages.   It is very easy to misinterpret e-mail messages, so be tactful and careful.