i filed my federal & state taxes, now how do i go back and submit my property tax refund minnesota

Since you already filed, wait for MN to process your return then amend it to claim the property tax refund. 

In your state return, continue to the screen Other Forms You May Need and select Property Tax Refund(M1PR)

Or, 

Complete and send  Form M1PR, Homestead Credit Refund. 

For filing details, see the Form M1PR instructions. If you are eligible for the Special Refund, see the 2024 Form M1PR-SR, Special Refund for more details.