Yes, it could increase your refund, and yes, if you filed and wish to include the additional information, you would need to file an amended return. Box 5 on a 1098 from a mortgage lender reports PMI which was eliminated as a deduction earlier this tax season, but then reinstated after the tax season was underway. 

You may add the form to see if it impacts your refund (it might not for various reason, such as you not being able to itemize). If it does result in a better refund, remove the entry and amend the return. Remove the entry either way so that your account has your tax information the way it was truly filed. (or start an amended return and delete it if there is no impact your tax)

CLICK HERE for TurboTax Tools and Tips about Amended Returns

https://ttlc.intuit.com/replies/6603004