Apologies for the delay in reaching out to you. I have tested this issue in my lab by creating a same test application as mentioned in your query.
After the application is created i was able to edit the url ( saml sign in ) information successfully, i dont see any challenge in updating those values if you have created the application yourself.
Have you tried logging off from the current session and testing the issue again.
Reference: check for the permissions who can edit the application configuration
If you have any other questions, please let me know.
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