Hi SebKaine, for this case untrim steps are like this:

Select this face:



Use Edit > Separate to break it out to be by itself and not joined:



Now to untrim you will need to select all the edges that make up a trim boundary. A quick way to do this is to select one edge:



Then use Ctrl+A to select all edges:



Then use Delete to remove that trim boundary and recover the underlying surface:



- Michael