If I understand you question correctly. You did the following: a) used Text object and typed some text, b) used some other object like Rectangle, c) now you would like to change the text.

One solution is by using Layers. For each text object create a new layer. When you need to change the text, erase the text with Eraser on that specific layer and type new text.

There is one more option, but it is useful only right after typing the text, because it involves undo action. Do the following to demonstrate a solution:

1. Create new image (to easily follow).
2. Make sure History pad is visible (View | Tool Windows | History should be checked).
3. Click on Text object from Tools and click on canvas and type some text.
4. Look into History pad. What do you see? You should only see "New image". But now do some other task, like click somewhere else on an image.
5. What do you see in History pad? There appeared two new entries Text and "Text - Finalize".
6. What actually you can do now (losing last editing if any) is using undo (like pressing CTRL+Z or using Undo button from toolbar). You need to undo the "Text - Finalize" action. When it is undone then History pad "Text - Finalize" entry gets grayed and italic.
7. Now the trick. Press CTRL button and click on text on canvas. What happens? You can reedit the text. Cool isn't it! This is a feature implemented by one of the students in Google Summer of Code project few years ago.