The title of the deceased can be edited even after the matter has been activated (and therefore when the first and surnames are locked).
In order to edit the title of the deceased, you'll need to go to "FILE..Open..Browse the full list of matters"
Scroll down to locate the matter you wish to modify. On the "Deceased..Name" page you can choose a different title for the deceased and save it by pressing the relevant button at the bottom of the screen.