See the instructions on the move plugin page:
https://www.dokuwiki.org/plugin:move
It's basically very simple. Go to the page you want to move. Then go tp the admin page and click on:
Page/Namespace Move/Rename...
In the top box, you can either select the new namespace from the drop down menu, or type in a new namespace, You can also change the name of the page. You can then click the preview button to see what will happen. When you are satisfied, click the submit button.