Heeelp :( Replacing text

my HTML is :
<li> this is MY   first      </li>
<li> this is MY   second   </li>
<li> this is MY   third     </li>
<li> this is   MY   fourth    </li>
<li> this is MY fifth    </li>

i want to change it to :
<li> this is HIS   first     </li>
<li> this is   HIS   second </li>
<li> this is HIS   third     </li>
<li> this is HIS fourth   </li>
<li> this is HIS   fifth      </li>

HOW ??