Google webmaster console, bing or another 3rd party service might require you to upload a file or add a code inside <head> tag in order to verify your site ownership. This can easily be achieved in one of the following ways.
If you receive a file you need to upload into your site root folder, it is very important that you upload it into "public" sub-folder and not website root folder, otherwise it will not work.
If you need to insert some code into <head> tag on your site, best way to do it is to insert it from admin -> appearance -> custom code -> Custom HTML & Javascript panel. This way your added code will not be overwritten with future updates and you won't need to modify any files directly.
Alternatively you can insert this code directly into common/resources/views/framework.blade.php file, however this is not recommended as changes to this file will be lost during updates.