How to add user to project administrators?

My goal is to change the default branch for a project to another branch.

Documentation tells me to use the Administration menu item of the project and switch it there.

I am Organization administrator but that’s the only items I see when I open a project:

No Administrator menu item.

So my question is twofold:

And welcome to the community forum!

That means your are not administrator of this particular project. But you can still regain your administrator right on it since your are an organization administrator.

For this you need to go in your organization’s Administration -> Projects Management page. And from there you can Restore Access to a particular project to have your administrator rights back:

Once you have your administrator rights back on this project you should be able to change the default branch as describe in the documentation.