I have the SharePoint online like
Company123.Sharepoint.com. I want the URL to be
What is the best way to change the URL?
This is unfortunately not possible, your only option is to create a new tenant. You can give your vote for this to be implemented here: https://office365.uservoice.com/forums/273493-office-365-admin/suggestions/10285887-make-is-possible-to-change-tenant-name-in-office36
This is not possible to change the tenant URL.
Unfortunately, it isn't possible to change the SharePoint domain name for your organization in Office 365. For example, if the name of your organization changes from "Contoso" to "Fourth Coffee," you can't change
To use the domain name
fourthcoffee.sharepoint.com, you would need to purchase a new Office 365 subscription and move all email, files, and any other data you want to keep to the new subscription.
Source: Change your SharePoint domain name.
Similar UserVoice: change tenant name/url.