You can upload or modify your logo by connecting to your Twenga Solutions account and by clicking on the roll-out menu on the top right of your interface just next to the name of your site and logo.
Place the mouse on "LOGO", then you will be able to click on "Modify":
You will then access the window allowing you to upload or modify your logo:
Your logo will be put online within 72 hours. If passed this delay your logo is still not updated, or if you are unable to upload your logo, contact the Support via our contact form, specifying the name of you site (or client number) and attaching your logo in the 87 x 26 pixels format.
This article might also interest you: Where can I find my client number/client ID?