With Vivid, you can easily display a different logo in the fixed navigation bar.

Step 1
Upload the new logo you want display in the fixed navigation bar to your Media Library.

Step 2
Copy the URL to the logo you uploaded. You can find this in the Attachment Details windows from within the Media Library.

Step 3
Go to Appearance > Editor > header.php

Note: Many security plugins will remove this option from the WordPress Admin. If you do not see the Editor option, disable any security plugins you are using or edit the file via FTP, SFTP, or in your hosting cPanel.

If you are using a multisite installation of WordPress, you will need to edit the file via FTP, SFTP, or in your hosting cPanel.

Step 4
On line 199 in the child theme's header.php file, remove https://vivid.divisoup.com/wp-content/uploads/sites/57/2018/03/logo2.png and replace it with the URL you copied for your logo.
Step 5
Save your changes. Be sure to clear any caches installed on your site and in Divi > Theme Options > Builder.