Hi Mark, here's a video showing how to use untrimming to remove a hole in your particular model:



So a couple of things to note there - first of all notice how I've zoomed in a ways so that the side wall pieces that you want to remove are not too small on the screen? That's important so that you select them more easily.

Then I do one click to select the full object. After that initial click, when I move the mouse around you can see that faces and edges are highlighting - those highlights are showing you want sub-object will get selected if you do a second click to drill-in.

Then I do the second click to drill in to one of the side wall faces, then I do a window selection to grab all the other side wall faces (once you have selected 1 face or 1 edge, then window selection will target faces or edges of that object after that).

Then they are removed by pushing the Delete key, that leaves a hole on the top face that is not attached to anything else, and holes like that can also be removed by delete, so I drill in again to select that top edge and use Delete again to remove it - the result is the full top surface is recovered.

- Michael