Sure, you can Edit the application, in PowerApps, go to Apps on the left menu, then select My Expense application and you can clic on the Edit button:
Unfortunately, you will have to review the application in order to find the specific things that you want to change, the application has several screens for different purposes. From a brief review from the application, I can see on the Create new Expense screen that the approvers are coming from one of the fields that you need to indicate:
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