Create a new Swatch with the color you want. Click on this new swatch in the Swatches panel. Then Command- (or Alt-) Click on any additional Swatches in the panel that you want to have replaced. Now go to the flyout of the panel and select Merge Swatches.
...I want to redefine it with another color using the eyedropper tool.
This is not possible. Pick a color with the eyedropper and create a new swatch. If you whish you can delete the unused swatch or if you find it faster, merge it with the new one as Tom said.
Europe, Middle East and Africa