Domain switch problem

I have finished creating my website with wordpress, and initially, as a host I used altervista. So I have a site with https://www.myname.altervista.org domain. 1 year ago, I bought a domain using register and to create the site I used website ( one of the register tool to create sites ). Now that I’ve created the site with wordpress, I want to use the domain of register on this new site . Obviously, in register, I have to remove the domain from the website tool. Once this is done , how do I use my own domain, instead of the altervista?