TeXt use as logo, you can change it by replacing _includes/svg/logo.svg with your own logo.
TeXt recommend RealFaviconGenerator for generating favicon.
Open RealFaviconGenerator, click “Select your Favicon picture” and then choose your favicon picture.
After settings you need tell the generator where you would place favicon files, TeXt default put them to /assets.
It’s recommended to put favicon files at the root of your site.
If all are done, just click “Generate your Favicons and HTML code” to the last step.
Download favicon package and unzip it to the path you just set, and replace the HTML code in _includes/head/favicon.html.