How do I change from personal to business account

Am I able to use the same email, same user name, and the same everything else? I have a premium account

Not completely! I have more. Questions. My personal premium account ends October 27 this year. Do I wait ‘till then to change subscriptions?

So this is an annual plan, correct?

Yes, when you cancel it, the subscription will usually remain active until the current billing period ends. However, if you live in one of the countries listed here, you may be eligible for a prorated refund:

https://support.microsoft.com/en-us/account-billing/countries-with-prorated-refunds-for-microsoft-subscriptions-38c81df4-10c1-f3eb-8d2f-b04f980c435f

You can sign up for a business plan now if needed. You do not have to cancel the personal plan first. The only concern is that you would be paying for both subscriptions at the same time.

Keep in mind that personal and business plans operate in two separate ecosystems. Changes made to one subscription do not affect the other.

I am having trouble signing into certain Microsoft apps with a personal account and I thought business might be better because I am looking for another job in customer and client support. Dynamic looks something I should learn

So you are looking at the Dynamic product? If so, yes, that's not a business product, so you won't find it in the personal product world.

But just to be clear, if you sign up for Microsoft 365 business plan, it does NOT include Dynamic, it requires its own additional license. So you can keep your current personal 365 plan, then just by the Dynamic 365 alone and use the business username to use that product.