One of the most asked questions and there has been recent updates regarding how to update the content/emails in journey.
Option 1) With the latest update, in the active/running journey you can hover on top of the email activity and click on View and Update activity. Once you're in the dialogue box, please reselect the same email and hit save.
Note: The hover function might take sometime. Please refresh your browser if it takes too much time.
Option 2) The journey emails are essentially triggered send emails, so another way to update is to navigate to the Interactions > Triggered Emails in Email Studio. You'll then see Journey Builder Sends folder and you'll have to navigate to the right journey and right version number. Then it is matter of choosing the right Trigger Definition where you pause, publish and restart that email trigger.
Note: Pausing, publishing and restarting the trigger will put the
contacts in queue and won't affect the journey as long as you restart