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