I did it about a month ago now without much effort at all.
Bought a domain from within WA (it was a different name to my siterubix one.) then transferred it over using Kyle's training video.
Which is in level 2 of the training. I was a premium member by then.
Are you a premium member?

To do that, you first must buy a domain as Marrion Black has said. To buy a domain, go here: https://my.wealthyaffiliate.com/websites/domains

You must check if the name you want for a domain is available. And that name may be different to your SiteRubix subdoman name. For example: if your siterubix subdomain name is: awesome.siterubix.com, chances are that it might not be available as awesome.com

You can search for all the names you want until you find one that you like the most. To come up with some ideas you can go to:
https://www.namemesh.com/

Once you have bought you domain, you can then transfer all the work you have done on your siterubix site to your own domain following these simple steps: How to Transfer a SiteRubix Website to Your Own Domain

You can't change the site rubix to dot.com but you purchase a new domain it will give you an option to transfer your site rubix information to the new domain you bought. I think that's pretty cool.

You'll need to buy a domain name that's available. It may be different to your SiteRubix subdoman name.

Hi, just go to site rubix and chose site domains button and over there it is pretty much just few clicks and you done if you will still have issues let me know. I will be glad to help.
Mark