Once you register a domain name, you own that domain for the length of time that you specified, although you can potentially sell it or to transfer ownership to someone else.
The good news, though, is that if you want to buy a new domain name and to use that one instead, that’s not a problem. You’ll simply need to follow the same steps you followed when you purchased your first domain name and point the new name to your webserver.
Unfortunately, that’s not always easy because of the sheer number of domain names that have already been registered. You’re unlikely to find a dictionary word as an available .com domain, for example, and so you’ll need to be more creative. This is where that domain checker will come in super useful, because you’ll find yourself entering all sorts of combinations until you find something that’s available.
Finally, while it’s true that you can change your domain name, it’s best not to make a habit of it. After all, the more you use your domain name, the more powerful it becomes. Every time you change it, you lose a bit of the value that you’d worked hard to instill in the old domain name.