Firstly, you will need to convert your logo file to a favicon.ico file. To do this, bring up the Favicon & App Icon Generator https://www.favicon-generator.org/
Click 'Choose File' to select your logo. Make sure "Generate only 16x16 favicon.ico" and "Maintain Image Dimensions (don't resize to be square)" are the only 2 fields selected. Then, click "Create Favicon".
The converter will generate a favicon.ico of your chosen logo. Click "Download the generated favicon" to download the converted logo in favicon.ico format.
Once you have your converted favicon.ico logo, navigate to the HM Admin website. On the left side, click on "Apps". Click into "Content" under the Harvest Pro 3 apps area.
Click on the "Assets" tab at the top. You should see an "Images" folder, with a favicon.ico file inside. Edit this file by clicking on the edit icon to the right side.
Once inside the "Edit Asset" pop-up, make sure the name of the file is indeed "favicon.ico". Choose the converted favicon.ico file you generated earlier and click save.
This will successfully update your favicon.