Yep, easy: open the Swatches palette (Window>Swatches), double-click the swatch you want to edit and, well, edit its contents
Note: you can do it if the swatch is in your open document, but you can't double-click and edit swatches in swatch libraries (unless you open the document that contains the library, but that's another story). But if you click on a swatch in a library palette, it's automatically copied to your current document and you can edit it through the Swatches palette.
Jeez... I hope it all makes sense