Reading xml

I have a xml file

<idioma>
  <ingles>
     <cat1>value1</cat1>
     <cat2>value2</cat2>
  </ingles>
  <espanol>
      <cat1>value3</cat1>
      <cat2>value4</cat2>
  </espanol>
</idioma>

how can I replace the value of cat1 with de function replace() ?