u can set that the value for each field to be the existing info of the user, something like this (you will need to change the name for the variables, maybe, according to the name of the fields in tha db)
echo '<form method="POST" action="?pag=newAdd">
<b>Name: </b><input type="text" name="userName" value="'. $name . '"><br /><br />
<b>Address: </b><input type="text" name="userAddress" value="'. $address . '"><br /><br />
<b>Phone: </b><input type="text" name="phoneNumbr" value="'. $phone . '"><br /><br />
<input type="submit" value="Send"></form><br /><br />';
of curse u need to validate if it is an existing user and have all its related info ready to go... cheers.
also, i suppose that u already know that it is not the same to update info than adding a completely new row, note that im taking for granted that you have the users info on a db.