How to change this image?

it’s the background image of your header area.
it can be modified in your style.css file.
be sure you are using a a child theme before making any changes.

#header {
height: 10em;
padding: 0;
background-image: url('https://www.lasalonbianca.com/wp-content/uploads/2013/12/header5.jpg');
background-repeat: no-repeat;
background-size: 95.6em 10em;
}

replace this string of code
https://www.lasalonbianca.com/wp-content/uploads/2013/12/header5.jpg

be sure you replace the image with something that is same dimension to preserve the overall look.

Do you know how to get to this coding? What do I have to click on the side bar? Is it in Appearance?

to make the change properly please be sure:
1-have a child theme set up
2-the change is made in the child theme CSS file.
3-theme files are accessible in the dashboard under APPEARANCES in the editor section or via FTP.

alternately, if you do not need to make changes to any of your theme templates, you can use a CSS editor instead. one is available in the free Jetpack plugin using the Custom CSS module.
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i don’t know how your theme has been set up in your dashboard so i can’t tell you for sure which option is the best for your situation.

WP-Admin -> Appearance -> Editor -> layout.css

Line 566 – that is where the image URL is that you need to change.