1. Open the conversation, scroll to the bottom, and select ‘Reply.’ (note: you can also edit the subject line when choosing to forward or reply-all)

2. Select the drop down arrow next to the Reply icon.

3. Click ‘Edit Subject.’ A compose window will appear in the bottom, right-hand corner of your screen. Make the desired changes to the subject line, then send or save the draft. The previous conversation thread will still be included in the quoted text (for context), but a new thread will be started under the new subject.