My personal advice on this will always be: do it on computer.
PC is better than mobile. But that’s outside of the thread, so moving on…
Basic tools you need to edit/create games on CS are
- Text editor. I’m not familiar with iOS, but I think Atom text editor is an alternative you can see.
- Web browser. As per recent development on browser issue, you can now use any browser to open relevant files (instead of relying on firefox). Check out the link I’ve included.
Now, to answer this
my personal opinion would be doing it offline. The benefit of not relying on the internet is immense as you don’t have to do repeated upload-download cycle for every update you want to test—granted, this might be not the case since the new version of CS requires you to “upload” your script, but it’s still better than the online alternative.
As such, download all your .txt files and work on them locally.