With nothing selected in the artwork....

• Highlight a swatch in the Swatch Panel
• Choose Select > Same > Fill Color
• The objects using that swatch for that fill color will become selected
• Do the same again, but this time choose Select >Same > Stroke Color to find strokes using that swatch

Note I suggest using Fill Color and then Stroke Color rather than Fill & Stroke Color because this will better target the color use. If you have objects with varying fills, but the same stroke color, Fill & Stroke Color won't pick up those objects. By specifically targeting the fill color, then the stroke color, you ensure you get all uses of that swatch color.

With nothing selected in the artwork....

• Highlight a swatch in the Swatch Panel
• Choose Select > Same > Fill Color from the menu
• The objects using that swatch for that fill color will become selected
• Merely click the swatch you want to use to replace that color
• Do the same again, de-select all, click the bad swatch... but this time choose Select > Same > Stroke Color to find strokes using that swatch

Note I suggest using Fill Color and then Stroke Color rather than Fill & Stroke Color because this will better target the color use. If you have objects with varying fills, but the same stroke color, Fill & Stroke Color won't pick up those objects. By specifically targeting the fill color, then the stroke color, you ensure you get all uses of that swatch color.