how do I replace a word in a file using DOS commands??

Just go into the editor (edit.com) and change whatever. I have never seen any version of DOS (except maybe the really old ones) that do not have EDIT.COM among all the normal files like the command interpreter. From what I am aware there isn't an actual command besides edit.com that will do any kind of file editing of that sort in DOS or Windows.

To use EDIT.COM in DOS

1) Go to the directory with EDIT.COM in it (usually in the root of the C: drive or in C:\DOS\ or some similar directory)

2) Type "EDIT $FILEPATH" (excluding the quotation marks of course), where $FILEPATH is the path to the file you wish to edit (example: "EDIT A:\HOMEWORK.TXT" excluding the quotes)

3) In the MS-DOS Editor, navigate to the word you wish to replace, highlight it, and type a new word in. Then, click on File > Save and then File > Exit (or, if you don't have a mouse, use Alt + S then Alt + X)

4) Your file is now edited.

Note that this does not work for formatted documents (.rtf, .doc, etc.) and I don't know if this works for documents created with VIM or other UNIX editors; if it's plain text, this method will work.