Do you mean on the back side of each page? That's a function of the
printer, not Word. Some printers can do this - it is called duplex
printing.

If you do not have duplex printing you can do it as follows:

1) Select Odd Pages in the print dialog box.
2) Print the document.
3) Take the printed pages out of the printer, flip them over, and put
them back in the paper tray.
4) Select Even Pages in the print dialog box.
2) Print the document again.

Turn the monitor around?  :-)  Sorry, couldn't resist!

I assume you mean that you want the document to print with page 2 on the
back of the sheet that has page 1,  page 4 on the back of page 3, and so on.
That's called "duplex" printing.

If your printer can do it automatically, there will be a setting, somewhere
in the dialog you get by clicking the Properties button in the Print dialog,
to turn it on and off.

If the printer doesn't have the mechanical ability to do it, you can tell
Word to print all the odd-numbered pages, wait while you manually turn the
stack of sheets over and insert them back into the printer's input tray, and
then print all the even-numbered pages -- a process called "manual duplex".
There's a check box for that in the Print dialog if your printer can't do
automatic duplexing.