Go to the wiki page on the source site in one browser tab / window. Now there's two ways to continue:
If you see two toolbars, as in this picture
click the text box below to activate the toolbar, then click on the last icon on the first line: . This should open a new window, containing the wiki markup. Press Ctrl-a (select all) followed by Ctrl-c (copy).
If you don't see any toolbars, you can just copy the contents of the big text box (Click on it, then press Ctrl-a [select all] followed by Ctrl-c [copy]).
Go to the page you want to overwrite (create one if necessary) in the destination site the same way as above, but press Ctrl-a [select all] followed by Ctrl-v [paste] (instead of Ctrl-c as above)
Press OK in the destination page to save changes. You have now replaced the contents in the destination wiki page with that of the source page.