Google indeed doesn’t accept subdomains, but that doesn’t mean you need to buy a domain name and premium hosting.
I strongly recommend to anyone who is serious about their website to buy a domain name. It’s only like $10 per year, and enables many extra possibilities with your site in addition to looking a lot more professional.
Premium hosting is not required though. You can use your own, custom domain with free hosting as well, and we have no restrictions on you placing advertisements on your own site.
Step 1 is to decide if you actually want premium hosting. If you’re just looking for a way to use AdSense, you don’t need premium hosting. If your website is getting popular and you need to be able to handle more traffic, then premium hosting is a good idea.
Step 2 is to ask iFastNet to migrate your site from free hosting to premium hosting. They can do that for you easily.
Step 3 is to add your own domain to your premium account at iFastNet. The easiest way is if you link it to the same directory as your free subdomain. That way, you don’t need to move any files at all.
Step 4 is to just login to the WordPress admin area on your free subdomain and update the website URL there. If you completed the above steps, it’s as easy as updating the setting and it will just work.
As you can see, there is no need to do any migration yourself!
And if you do ever need to migrate your site, please do not use backup/migration plugins. They often don’t work, and leave you with a corrupted site. Both our website migration guide and the official guide from WordPress do not reference any migration plugins, because you should not use migration plugins.