I hope you are well today and thank you for your questions.
I want to upload the latest version of Jevelin, however I’ve added some custom CSS to my theme (in Appearance > Custom Code)
How can I be sure I will not lose these changes.
You won’t lost it after updating theme to latest version but just to be secured copy that custom CSS code in the text file before updating the theme so that if anything goes wrong then you can recover it.
And is worth creating a child theme at all?
Yes it is very useful. get more information about it on this page http://freewptp.com/why-and-how-to-create-wordpress-child-theme/