Well, you sort of don’t. It’s not really for that. There are potentially ways of doing this using the Process List action (which will create a different list for you at List Shifter’s Processed List output), but again it’s not straightforward.
Now, there’s nothing keeping you from, for example, setting List Shifter’s Custom List to be List Shifter's Original List :minus item (some item), but this isn’t exactly what you want as it’s not addressing that item by its index number (Bubble makes this difficult).
For a more straightforward approach to manipulating lists in the way you desire, @dimeprocessing, you might check out the various “RAM List” actions in Floppy which can be used to (for example) change or remove a specific item in the list held in Floppy’s RAM List. See the post here over in the Floppy thread and the video linked in that post!