It is possible to download only the modified files when upgrading to a new version, so that if you know that you didn't edit the code in those files, you just overwrite those files and leave the others, or if you did edit code in lots of files, you only have to upload the modified files and repeat edits in just those files.
There is a file that you can run to update automatically, but three times now I've started that and given up when I was given options I didn't understand. I've always done the whole upgrading process manually because I think I know what I am doing and can restore old files and start again if necessary.
It's still very nerve-wracking
