What is the best way to edit XML file data?

I have some xml files from an external client and i would like to edit the data within them NOT the xml parts.

Does anyone one know the best (preferably free) way to achieve this?

I have tried excel but I do not have the xslt style sheet and it will not let me save the file once I edit it.

Any ideas welcome.

Since XML is text, any text editor will work fine. With XML, "data" can be just about anything in the file, including the structure, so what you will need to edit is entirely dependent on your application.

In general, however, anything non inside angle brackets (< and >) is data, and also attributes on the elements (the things in the angle brackets) is also often data.