Symbol Shifter

-Changes stacking order of symbol instances in a set

To move symbol instances, drag in the direction you want the symbol instances to move.

To bring symbol instances forward, Shift-click the symbol instance.

To send symbol instances backward, hold down Alt (Windows) or Option (Mac OS) and Shift-click the symbol instance.

SOURCE:
https://helpx.adobe.com/illustrator/using/symbolism-tools-symbol-sets.html