3rd Way to Create a Site Icon
The third way to create a site icon is with the use of plug-ins. You can download a plugin specifically for creating a site icon. Go to Plugins > Add New
Enter the words Site Icon or Favicon in the search bar for new plugins.
Below is one option you can consider - Site Icon Pro. Be sure to check, however, if a particular plugin has been tested with your version of WordPress and your theme.
Another Plugin Option
Since you have chosen the third option of utilizing a plugin, in order to save on the number of plugins you have on your website, because as you may or may not know, the more plugins you have installed on your site, the more possible performance issues you have may. So, to keep the number of plugins down to a minimum, you should consider plugins that perform multiple functions.
You could consider installing a plugin for adding code to a header or footer. Go to Plugins > Add New and search for Header and Footer. Below is an good choice.
This plugin will take the guesswork out of knowing where in your HTML pages to paste HTML code. This plugin will automatically add HTML code to your header and footer.
There will be many occasions where you will need to add code to your header or footer and with this plugin you can easily do that without downloading a plugin each time you need to add new code to your <head> section. One plugin and all you do is copy the code that you need and place it within the space easily marked using the plugin features.
Again, be sure the header and footer plugin are compatible with your version of WordPress.