Layer selection
In order to remove or flip Layers, you will need to select them first. To do so, click on the area next to a Layer's name. It will be highlighted in blue when selected:

It can be useful to select several Layers at once. To do so, hold [Ctrl] or [Shift] when clicking on their names. The selected Layers will then appear in blue:

You can also select Layers through the stack's contextual layer menu which can be opened with a right click on a Layer:

Several options will then be offered to you. You will be able to select All layers, Active, Inactive or Empty Layers. You can also choose to Invert the current selection.
Finally, the Group sub-menu will allow you to select layers from the same Color Group (All, Active, Inactive or Empty Layers), as well as Invert your selection. This notion of Color Group will be explained later on.
Selecting multiple Layers will allow you to add new Instances and Exposure Cells to all of them at once.