Hi there!
If you are using the personal plan and have 1 website license, then you can use it for your main domain and sub-domains also.
You can use the license key on your localhost too as the localhost doesn't deal with the license key.
Yes, you can change domain name on your dashboard, as you can see an edit icon there. However, we discourage that, it will cause no trouble though. If you really need to build your site in a test environment then you can use your localhost, which is safe as well.
Best Regards