How to change the text (and the title, sub title, sub foot, foot) and to store these settings?

You can set all these from Plot->Layout->Edit... as shown below.

Note: In order for the text not to be overwritten the next time a curve is plotted, you must go to Plot->Layout->Text Refreshing.... In the dialog box shown below make sure the check boxes are unchecked as shown below. If they are unchecked, the text will not be refreshed and therfore the text you have typed in the dialog shown above will not be erased.