how to modify an xml file and add an inner node?

I have this xml file:

<xml>
<segmentrecorddb>

...

</segmentrecorddb>
</xml>

I want to write a sh file that will add an inner xml node

meaning the file will be:

   <xml>
    <segmentrecorddb>
    <myNoed>
    </myNoed>
    ...

    </segmentrecorddb>
   </xml>

how can I delete the first two lines

add the 2 lines and add