Updating Theme *Newbie*

I’ve read through several posts on here and am still confused.

I’m trying to help a friend out with her WP site so this is my first time using WP.

I’m trying to update her King Size Theme, I followed the directions but got confused where it said “Upload to your server where WordPress is installed, into the “../wp-content/themes” directory”

I’m assuming that means go to her dashboard > Appearance > Themes > Add New > Upload

2 things

1st off I’m confused when you say customization will be lost, do you mean if I changed any of the styles on the css file?
Can I copy the CSS file and then after the update just go back and paste it in??

2nd I’m getting error “Unpacking the package…

Installing the theme…

Destination folder already exists. /home2/gallen45/public_html/wp-content/themes/kingsize/

Theme install failed.”

And lastly, none of her content will be lost will it?

I tried finding a # to call WordPress for support but couldn’t.