There are 3 ways you can retarget subscribers with a newsletter.
1. Go to Campaigns, choose the campaign you want to retarget and click View report. Click on the tab Activity and navigate to the group of subscribers you want to target, for example subscribers that didn't open the email. Click on the green button next to Show and select Unopened. Click Actions, then Copy to group, give the group a name and click Copy. You can also select an already existing group. This will create a new group with all subscribers that didn't open that particular email. You can then create a new newsletter (or copy one) and use the group as the recipients.
2. Go to Subscribers, choose a filter, go to Campaigns, click "was sent" and "was not opened." Then click any campaign and select the campaign of your choice. Then add a condition that says "Groups" "belongs to" and select your group. Then save the segment and give it a name.
3. Create a new campaign like you're used to. Once you've created your newsletter and landed in the Recipients step, navigate to Advanced. Click "Campaigns" "was not opened" and the campaign of your choice. Then add another condition saying "Group" "belongs to" and the group you've sent that particular campaign to earlier. Then continue as usual. This is the quickest method to retarget subscribers.