How to delete read emails from iCloud

See this article https://support.apple.com/kb/PH12795?locale=en_US&viewlocale=en_US

Erase messages using your iOS device

On an iOS device set up for iCloud Mail, you can free up iCloud storage space by permanently erasing iCloud email messages in the Mail app. 

1. Delete messages, which moves them to the Trash, by doing any of the following:

• • Delete a message that’s open: Tap .
  • Delete a message without opening it: Swipe from the right to left over the message title in the message list, then tap Trash.
  • Delete multiple messages: While viewing the message list, tap Edit.

1. To permanently erase all messages in the Trash and free up iCloud storage space, tap Trash in the list of iCloud mailboxes, tap Edit, then tap Delete All.