If you want to change the logo for an agency then you can do that as explained in a comment on this thread How can I quickly set up Primero - #5 by irlawrence
If you want to replace the main Primero logo itself then you need to do something like
sudo -Hu primero bash
cd ~/application
RAILS_ENV=production bundle exec rake app:assets_precompile
and then restart the services. The default logo is logo.jpg and not logo.png b.t.w
