How to change your landing page domain
You can change the domain that your landing pages will be visible under by clicking on your account name in the sidebar and going to Your profile > Account settings > Landing pages. Here, you can edit the Account domain section.
The new name should only contain letters, numbers and dashes, any other characters will be automatically removed. Every domain name needs to be unique, so if another user is currently using this name, you won't be able to choose it.
Keep in mind that if you change your account domain, the URLs of any existing landing pages will change too. This means any old links you've shared will no longer work, so you'll need to update them with the new URLs.