Every recurring assignment is made up of one or more individual assignments which are created based on the recurring assignment's rules. You can edit the individual assignments within that recurring assignment to "tweak" them as needed, for example to delete ones when a volunteer cancels, or to adjust for a week when he'll be arriving an hour late.
Click on any non-cancelled recurring assignment to edit the individual assignments within it.
If this assignment is paired with a job slot, VSys will not let you edit most of the properties here.