Each cell of the SFD Selection Grid represents a system. You can select each one by clicking on it, and deselect it by clicking on it again.
If you only want to deselect one system, click on it and it will disappear; then click the “Create SFD Matrix” button. This will redraw the matrix without that system, but ALL other data is kept.
If you want to keep working on the same city but start afresh, then deselect all the systems, select the new set of systems and then click “Create SFD Matrix”. This will draw a new SFD Matrix, which is empty and ready for you to populate with data but the general information data you entered about the city will be retained.