You can modify an instance without affecting the symbol or other instances by first breaking the link between it and the symbol.

To release an instance from a symbol:

  1. Select the instance.
  2. Select Modify > Symbol > Break Apart.

    The selected instance becomes a group. The symbol in the Library panel is no longer associated with that group. After separation from the symbol, a former button instance loses its button symbol characteristics and a former animation instance loses its animation symbol characteristics.