The simple way to custom your site logo in Drupal8 is to upload your logo with the name logo.svg in your custom theme directory. The Path is something like : sites/YOUR-SITE.com/themes/MY-THEME/logo.svg. This will override the Drupal8 default logo.
The second way is from the Backend . Just go to Appearance->Settings->Global settings and in the “Logo image settings” give the path where your logo.png is.
Just change “YOUR-SITE.com” and “MY-THEME” with your custom folder names.