In Momento 3.5+ just tap the 'Restore Purchases' button on the 'Premium' settings screen to restore access to premium features. Ensure you sign in with the Apple ID you used to originally purchase Momento 3 or Momento Premium.
In Momento 3.4 or earlier just tap the 'Refresh' purchase button in the top left of the 'Premium' settings screen.
If Momento is unable to verify your purchase then this is because a valid receipt for Momento 3 or any of Momento Premium in-app purchases/subscriptions could not be found with your Apple ID. Please note that Momento Classic purchases are not eligible for the free upgrade to Premium.