Let's assume you have real MSWord footnotes, with references in the text.
Probably you inserted them by typing Ctrl-Alt-F, which put the reference
number in the text and the note at the bottom.
Let's assume you have Word2007 or more recent. In the References tab,
Footnotes group, click the tiny arrow at its lower right corner. Assuming
you have real footnotes in the document, the "Convert" button will be
available. Click that, and choose "Convert all footnotes to endnotes."
If you're asking how to insert an endnote rather than a footnote, type
Ctrl-Alt-D where you want the reference to be.