I would like to modify an attribute of a very long xml like this:

<element index="0">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
<element index="1">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
...

I need to add N the value of each index attribute. Say N=5. The result would be:

<element index="5">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
<element index="6">
    <subelement bla="asdf" />
    <subelement bla="asdf" />
</element>
...

What's the easiest way to do this? I presume it would be with XSLT, but I don't know how to do it.

Thanks!