Thank you for your help :
so these steps are working until the fourth :
when I "Switch to Edit tool" (two double-arrows), and when I double-click on the label already created, it doesn't turn to another colour, and it doesn't open up. However, the cursor is changing into an arrow pointed up.
So what can I do now ?
I precise that otherwise, when I have juste created a new text, if I want to change it, without waiting a long time, it works well !
So would the problem origin from layers active or not actives ?
I thought I was working with one layer for putting all my labels, but maybe not ... ?!
Thank you again for your next answer,