Hello,
So you would need to cancel your personal plan first, then subscribe to the business plan. They are two separate plans, there's no in-place upgrade.
During the business plan sign up, you can create a business username in this format: username @ businessname.onmicrosoft.com. Here username and businessname will be defined by you. And the domain onmicrosoft.com is offered to you.
Then you can work on bringing your own domain in once the plan is set up with above username. You would refer to here to add domain:https://learn.microsoft.com/en-us/microsoft-365/admin/setup/add-domain?view=o365-worldwide
Do you know who's hosting your domain?