I cannot change templates or patterns

Hello team,

We changed between providers. Our new provider has moved the website toerfietsleusden.nl from old to new provider.
Nou I want to change something in the heard.
Unfortunately this seems impossible. I cannot change templates or patterns of pages. Is it possible that one of you take a quick look to see what the problem is and how to solve this?
Sorry for bothering you and thanks in advance.

Kind regards

Hans Hagmeijer