Each theme can provide its own settings, but all themes have a common configuration. Click on the Settings link next to the Bartik theme to open the available configuration.
For now, skip over (collapse) the Color scheme option (we'll come to that later).
Inside the TOGGLE DISPLAY menu, you will see a list of checkboxes as follows:
By default, if the theme you have chosen to use includes its own logo.svg
(Scalable Vector Graphic) file then that will be used as the logo.
The default logo is a semi-transparent version of the Drupal 'drop' character:
Thus, when used overlaying the Bartik's theme blue header, it appears:
If you choose to un-tick the Use the default logo supplied by the theme option then you are given the chance to point to, or upload, an alternative.
The default shortcut icon, often referred to as the favicon, is also the Drupal drop icon.
Once again, if you would like to replace it with something else then the option is there.