A favicon is an a logo that we usually encounter on our site to improve our look on site has of own custom icon via a widget in a layout or settings of your site
Leverage Browser caching on favicon is one of the factors which usually get measured for professional & site like GTmetrix pages
First, open Blogger > Click on Theme menu > Click on "ctrl F "
> then look for the code below & change it
Replace the code with the given code below
Also do not forgot about changing your domain name in the below code.
Then Click the Save button