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Select existing to overwrite
This is the initial view of the existing disk partition layout.
Don't be confused or intimidated by my rather large number of partitions, a more typical one will probably have something like one to four partitions, depending on what is already installed.
If you want to reuse an existing partition, click the drop-down for that installation in the list. Then select the partition you want to reuse, and its details will be shown at the right side of the window.
In the Mount Point input box, enter what you want to use this partition for (it is / in this case). You must also click the drop-down for Customize... and then click the Reformat box, and then click Apply Changes.