Clear cookies on iPhone 6+
Navigate to Settings > Safari. Then scroll down and click the Clear History and Website Data button. This clears history and cookies

If you dont want to clear history, and just cookies then do the following:


Press Settings on the Home screen.
Scroll down to Safari. Press Safari.
Scroll down to Advanced. Press Advanced.
Press Website Data.
Press Edit.
Press the red circle with the minus sign next to the cookie you want to delete.
Press Delete.
Repeat step 7 until you've cleared all the cookies you want to delete, then press Done.