To update your profile details on Foursquare City Guide, navigate to foursquare.com/settings. Here, you'll find options to modify your profile information, adjust sharing preferences (including connections to Facebook and Twitter), manage privacy settings, customize notifications, review connected apps, and oversee managed pages for businesses. Keep in mind that changes you make here will reflect on your Swarm profile too.
How can I change my username?
If you link your Foursquare account to your Twitter account, we'll automatically use your Twitter username as your Foursquare username. You can only change your Foursquare user ID by linking your Twitter account. Head over to Foursquare.com/settings/sharing and connect your Twitter account.