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.