I just did this successfully, but I also experienced some cases of it not seeming to want to accept / save input.
1. The UI is rather poor, I think in some cases when you press “Save” it actually saved the change, but it does not give any confirmation. You can find out by navigating to another page and then back to the settings page you are looking at. If your change is still there then it means it was saved. (This was the case for me.)
2. Possibly there is a browser compatibility issue, sometimes poorly-written UIs won’t take form posts or otherwise have problems with particular browsers. Try using a different browser or ensure that your current browser is upgraded.
Maybe that helps…