Above your Matrix heatmap you will have a title/label if you have named your risk list. If your process was created using one of our templates, the title is likely to be Risk List.
This cannot be changed on the graph screen.
To change the wording, go to the List Item Editor and highlight the text and delete the existing label and replace with your own.
To change the colour, right click on the label text.
The list name will be updated on both the list view and the heatmap view.