I am probably misunderstanding the question and what you mean by customize. I'll answer it based on how I read it, but if I have something wrong, please explain more.
To customize or change the content on a page, you go to the page and click the Edit button at the top.
If that doesn't work for you, then it is possible that your institution is using instructional designers that design your course and you are allowed to teach but not modify the material. Another possibility is that your institution is using blueprint courses and locked the material so that it cannot be changed.
In either of those cases, you would need to contact someone at your institution. This is a global community of Canvas users and we do not have access to your institution's Canvas instance.
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