Using wordpad when Gnome is available is kind of counter productive. My reason is that I just don't trust an editor to correctly format a tabulated text file.

But, if you want to learn, first understand that wordpad adds a carriage return on the end when you select whole lines using the reverse arrow. This can cause problems later where you get that message about by saveing as a text file, you will lose all formatting.

A less risky method of selecting a line is as follows - place the curser at the beginning and with 'shift' held down, press 'end'. Then press the back arrow (not backspace) once so the CR isn't selected. Press ctrl-c (or ctrl-x to cut) and ctrl-v to place that line where you want it.

Beyond that, to change text is simply a matter of selecting all of the text in a field and then typing in the new value. What makes Gnome easier is that doing this in wordpad gets extremely tedious and can be quite confusing.

One thing that helps me is to open the "Find text" box and place the bottom of the box in the middle of the line above the one I'm editing (or the top that box in the the line below). Then as long as I don't scroll up or down, I always know I'm editing the right line.