Please make sure you are signed in to the Microsoft Store with the same Microsoft account that was used for the purchase in 2022.
Then open the Microsoft Store, go to Library / Downloads, and look for Diarium there. You should be able to reinstall it without purchasing again.
If the Store still asks you to buy it again, then the Microsoft Store does not recognize the current account as the purchasing account. In that case, please contact Microsoft Store support with your receipt, because the purchase/license is managed by Microsoft, not by me directly.
