Sharepoint - How to add logo in Modern Team Site
The below is the "official way to go", but at the end is a suggestion to check for a quick logo change.
Modern Team Sites can't be customized through the settings-menu. Instead, to customize Modern Team Sites, you need to upload required files and assets and apply the changes by executing the
ApplyTheme method. To specifically change the logo of a Modern Team Site, you need to use the Groups Graph API and define the logo in there.
See more about Applying custom theme to "modern" team site and SharePoint PnP including the method for logo changing.
There's a suggestion that you can change your logo by accessing the default logo at
Site Assets stored with the name
__siteIcon__.jpg. By replacing this file with an identically named filed you could apply your own logo to the site.