That is quite a big mistake on your part. Next time read instructions first. https://ronangelo.com/forums/topic/customizing-wordpress-themes/
The “Use a child theme before you customize” rule applies to all WordPress themes. http://wordpress.org/search/forgot+child+theme?forums=1
could you please guide me to update the theme without ruining my website?
Sorry, I don’t think there’s an easy way to do this. You may have to go through all your modified files to copy the changes you made.
If you only modified the style.css of the parent theme you could use this site to compare the difference between that file and the new file.