Normally, you can't do that, but if you create a vector layer and put the text into a vector layer, you can just click on it and edit it. You'll also need to save the image as .rli to not lose this ability.
Select, Delete, Edit or Move created Text
Image Editing for Dummies
After creating text on an image, how then do I go back and re-select, delete, move or edit previous texts I have already created? RealWorld paint v2013.1