Follow the steps in this help article to update your company logo in the upper-left corner of Rethink.
The following steps must be performed by the System Administrator.
1. Go into Setup
2. Go to Themes and Branding by typing it into the Quick Find search box, then selecting it.
3. Locate your company name from the list of themes, then choose Edit in the drop-down box next to that theme.
4. Click on the Brand Image box and upload your logo.
5. Make any other branding/theme updates as desired, then click Save.
6. Click Activate at the top to activate your changes (important).