I think there isn’t an option to select a license in the new version right now, so you can do it using the old one.
For editing cells and notebooks, you need to first enter edit mode, which is not enabled in some notebooks. To enable it, there is a sidebar with two buttons in the bottom right; the second one (a pen icon) is for editing. If the sidebar doesn’t show, try enabling desktop mode in your browser. Then, click the left margin of the cells to open the textarea for editing.
Thanks for the reply @nxrix, that’s right! And I just added a Tinker button to the top of notebooks when you’re in read-only mode and you’re not allowed to save changes to the notebook. This makes tinker mode more discoverable and you can now tinker on mobile.
One thing I noticed, though, is that I couldn’t find a way to tinker with my own published notebooks. There is only the Edit button where changes get applied, and a Republish button will show. I don’t see an option to discard the changes.