This post will be updated to link to a future post that outlines all the different Power App Portals areas. If you have spent more than a few minutes in Portals, you will quickly notice that there is no single place to update or change settings.

How do you purge the cache for a portal?
Navigate to the Power Platform admin center
https://admin.powerplatform.microsoft.com/
Click on Environments and select your environment
Select the portal, then click Purge Cache in the ribbon

edit
The purge cache button won’t always pop up the modal asking if you want to purge the cache. If you visit this page, select your portal site, click the Purge Cache button, and the modal will appear.

https://admin.powerplatform.microsoft.com/resources/portals