how to clear my old e-mail

What do you mean by 'clear my old e-mail', what exactly are you wanting / trying to do ?

If you've changed the primary email address on your account : What to do after you change your Apple ID email address or password - Apple Support

If you want to log out of an account tap on it in Settings > iTunes & App Store etc. and log out.

Do you mean to remove your account from your device? If so, then simply go to settings > accounts and passwords > select the account you want to remove > click delet account. If, on the other hand, you meant how to delete all your past emails on your device, simply do the steps named above, but re-add your account. Here is a link for more info: https://support.apple.com/en-au/mail. Hope this helps!