Purchasing I bonds with Refund

I've gone through the whole turbotax process to "Ready to e-file".  But am now thinking of purchasing I bonds with some of the refund.  How do I go back to actually purchase I bonds?  Going back to just reducing the amount to be applied to next year's estimated tax  doesn't give me the screen on what to do with the balance.  It just assumes I want a refund.  Also is it really true that you can't each purchase $5000 I bonds if filing jointly?  You're only limited to 1 purchase?  Thanks.