After system-zones are created, you can edit them by editing the sketch of a sketch-based zone or adding or removing spaces from a zone created by space.
To edit a system-zone sketch.
Mode panel
(Edit Zone).
To edit a system-zone by space
Mode panel
Zoning panel
By Space
.
Mode panel
Mode panel
To add/remove multiple spaces to/from a zone draw a selection box around the spaces being added/removed, or press CTRL while selecting spaces one at a time.
Mode panel
(Convert to Sketch-Based).