1. Open Gmail.
  2. Click the gear in the top right and then select Settings.
  3. Scroll down to the Conversation View section (stay in the “General” tab).
  4. Choose Conversation view on.
  5. Click Save changes at the bottom of the page.

Once you have done this, you should see email un-threaded view. You can switch back anytime if you prefer the conversation view to be on.