You can amend your Minnesota state return in order to change previously reported information. Log in to TurboTax, go to Tax Home, scroll down to your tax returns & documents, select the year you want to amend. Then select the Amend (change) return drop-down. Choose Amend using TurboTax Online. On the "Why are you amending?" screen, choose "The IRS or state contacted me" and "Continue," this will allow you to just amend the state and leave the federal return as it was.
Navigate to States > Minnesota to add the information for the credit.
In order to enter the information in TurboTax for the Renter's Credit, you go through the MN interview until you see the screen Other Minnesota Credits. The first option is Renter's Credit. Select this and continue. You'll need to confirm you meet some requirements, then you can enter information from the Certificate of Rent Paid.