How Do I Delete Individual Cookies From My iPhone?
Go to the Settings app on your iOS device and select Safari. Swipe all the way to the bottom and select Advanced. Then tap on Website Data. Swipe down and select Show All Sites. You can then tap Edit at the top right of the screen and delete all the cookies associated with an individual site.
If you only want to delete some cookies from a site and not all of them, go back one screen and turn Web Inspector to the On position. Plug your iOS device in to your Mac and open Safari on the Mac. From your iOS device, go to the website in question in Mobile Safari, then switch back to your Mac, go to Safari > Preferences > Advanced and check "Show Develop menu in menu bar". Then go to Develop and select your iOS device and the site that you have open. Click the second icon in the top left of the window (looks like a stack of cookies) and then click on Cookies in the sidebar. You should then see each individual cookie from that site, which you can then highlight and delete.