I hope, this is what you're looking for:
When you receive and open an email, tap the arrow that's showing to the left, then "Reply". When you do this, you can place your "cursor" everywhere in the received email message and change whatever you like. When you've finished your editing, tap "Send".
The IPad Forum is a community of members from all walks of life and from all over the world coming together to share our experiences and to help each other learn about this wonderful piece of technology the Ipad. There is a wealth of information already here in the many threads we have so doing some searches will answer most of the questions you have. If then you cannot find the answer you are seeking then by all means post your question. There are many friendly and informed members here only too willing to help you.