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You can do this two ways.
1. Change the swatch, which will change everything that uses that swatch:
"To replace a swatch, hold down Alt (Windows) or Option (Mac OS) and drag the color or gradient from the Color panel, Gradient panel, an object, or the Tools panel to the Swatches panel, highlighting the swatch you want to replace.
Replacing an existing color, gradient, or pattern in the Swatches panel globally changes objects in the file containing that swatch color with the new color, gradient, or pattern. The only exception is for a process color that does not have the Global option selected in the Swatch Options dialog box."
2. Select everything with the same stroke or fill color and then select the new swatch:
"To select all objects with the same fill or stroke color, select an object with that fill or stroke color, or choose the color from the Color panel or Swatches panel. Then choose Select > Same and click Fill Color, Stroke Color, or Fill & Stroke on the submenu." Once you have it selected, just set your color.
Illustrator help file & personal gratitude for these features.