For the logo, go to Settings > General Settings. The logo can be updated by either clicking on Update Info below the company name and editing the logo or by clicking on Manage Companies and editing each company and setting the logo for them.

For the favicon, go to Settings > Website​​​​​ and set the favicon in the Website section. If you have different websites by company, you will need to select each company from the dropdown on the top right of the screen to set the favicon for the website corresponding to that company.  ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​   ​   ​ ​ ​   ​ ​ ​   ​ ​ ​   ​   ​  

As far as I remember, the settings for company logo (using a contact's company logo instead of your own) is part of the published settings.

In Contacts, go to the contact, click on "Go to website" and check the page.  Click on "publish"

Hey Niklas,Check this forum answer https://www.odoo.com/forum/help-1/how-to-set-default-company-logo-167221