In the Segments part of the Subscribers section click to "Add Segment"
Step 1 - Give the Segment a name
Step 2 - Give the Segment a description (optional) Step 3 - Pick the which Group(s) you want to base as the data pool for your segment (this would match the audience that you sent your campaign to) Step 4 - Click to add the Dynamic Filter called "Opened Campaign". From the two drop down settings available select "Not Opened Campaign" from the first one. From the second drop down select the campaign you sent to, and which you want to resend. Save this filter by clicking on the "tick" icon
Click to 'Save & Close Segment'
Go back to the email in the Create and Send section and go through the sending wizard, selecting the Segment you made when you come to the 'Sending Options' step.
Benefits of using a Segment include:
The reassurance that you are only sending to Subscribers who (at the very second the resend campaign goes out) hadn't opened the original campaign, because a Segment is a means of saving a set of dynamic filters to apply at the point of sending our your email - rather than a physical set of actual Subscribers. Using the 'export to new Group' method means you may resend to Subscribers who opened you email between the time you exported the data from Track and the time you do the resend.
Not needing to make another Group to add to your account.
Being able to refer back to the Segment at any time to edit the selected past Campaign and filter for new data. For example, you could select the resent campaign and see who still didn't open the second send and decide on a best way forward to engage with those Subscribers.