Revising a number format in Outline Numbering in Word 2016

[Very Terry]

Hello,
I use Word 2016. I have created an outline numbering (multilevel numbering) and need to make an edit to one of the heading styles. I read in a previous posting that I should click on the Multilevel List button, then right click on the icon under Lists in Current Document. There I should be able to select Modify. But I don't get the option to Modify. (see attached).

How can I change the number formatting of one of the levels without messing up the formatting of all the other ones (or without breaking the sequential numbering)?