Here are some screenshots and better explanations of the steps:
Start the “Settings” application.
Go down to the third grouping of options and select the “Mail, Contacts, Calendars” option.
Scroll down the screen to the third grouping of options under Mail. You will see a “Organized By Thread” switch. To switch the off email threads, turn the switch to “off”. To turn it back on, turn the switch to “on”. When threaded emails is on, all emails with the same subject are grouped together for easier reading. When it is off, emails are organized by when the are received.