Can't edit permission JSON on AWS, despite it telling me I can

I am hoping to set a time limit on AWS permissions for my users.

But I cannot edit the policy JSON file, despite it saying I can:

Note the line You can still view or edit the JSON policy document.

But then it says read-only. Is there a way to download the JSON, edit it, then upload it?

Or is there another way to put a time limit on user permissions?

Looks that you are trying to modify "AWS managed" policy (you can see the small aws logo before the policy name) and that is not possible. You can create a new policy (will get a tag "customer-managed") and then you can copy/modify permissions