In Acrobat X, click on Tools. You'll need to open the Print Production panel which is not usually turned on: Go to the flyout menu at the upper right and check Print Production.
In the Print Production panel, click Preflight. The Preflight window opens.
At the top of the window, click the Fixes button (it's the rightmost of the three buttons—see screen capture). You'll see a list of fixes in different categories. Open the "Color spaces, spot colors, inks" section and select the Convert color to U.S. Web Coated SWOP v2 option. Click Fix.
Since you're permanently changing the file, you'll be prompted to save the file under a new name.