To change the base URL of a website, navigate to Power Platform Admin Center >> Resources >> Power Pages sites, select the site, and click on Manage.
https://admin.powerplatform.microsoft.com/resources/portals
Select the Edit icon
If the user has only the system administrator role in the corresponding Dataverse environment, we will get the below error.
You must be a Global Administrator or an owner of the Microsoft Entra ID application linked to this website to perform this action
Check the Required role and permissions
Then we tried with the global admin user having all the appropriate roles and permissions and were still getting the below error.
You need to have access mode as read-write and be an owner of the application linked to the portal to perform this action.
The global admin user was the owner of the corresponding application registered for the portal.
To fix it we refreshed the user’s account (we had PIM enabled for the Global Admin role), so maybe refreshing the user or giving it a try after some time helped in resolving the error.
Hope it helps..
Nishant Rana's Weblog 
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