Rename Root Node in XML

how can i rename a specific node like:

<group>

<a id="a"> t1 </a>

<b id="b"> t2</a>

</group>

i want to change <b> into <c>

<group>

<a id="s"> t1 </a>

<c id="c"> t2 </c>

</group>

please help me in this