When you make a change in the current article, the article ruler will display a red border and the statusbar will display the text 'Article not saved' or 'Autosave in x min'.
Once you have finished editing the article, you can store it,
(1) by clicking menu⁄database⁄save current article,
(2) by clicking on the 'store article' button, which is only visible when the article is being edited, and the mouse is hovering over the lower right part of the article (see screenshot below).
(3) by typing Ctrl S
(4) by clicking on the tree with the mouse,
(5) by selecting a different node.
After having saved the current article, you will see that the article ruler displays a normal border color again indicating that changes are now stored in the TreePad X database.
Any changes you make to the tree are stored instantaneously.