First, go to the design tab.
Next, click on the Edit HTML link.
On this page scroll down to the box that says Edit Template. About 7 lines down you'll see <head> you'll want to add the following code <base target='new'/> on the next line. See picture:
Lastly, click the Save Template button and you are done. Now, I will warn that you should probably download your template before starting incase something goes wrong.
(Right above the Edit Template section is the Backup/ Restore Template and there's a link that says Download Full Template. )
I hope this information helps!