There are two ways to re-edit an already created text box or image.
Both methods require gesture mode.

Method 1

1. Touch/Hold the part you want to edit.
2. The part is then selected for editing.

Method 2

1. Tap one more time while selecting the lasso tool. 
2. Enable ‘Textbox’ or ‘Image’ option
3. Select textbox or image using lasso tool and then you are able to edit.