Can’t modify theme?

I listed this under “themes” but then realized it was really more troubleshooting.

My host automatically installed WordPress for me. I downloaded the them Nishita and installed it. I thought that when you are hosting the WordPress software on another site you could edit the theme. But when I edit it – design:theme editor:header.php – and click “Update File” it just refreshes the page, deletes my edits, and doesn’t let me actually change anything. What should I do?

https://www.giggleboxphotography.com/blog/

Thanks!