There is no way of upgrading from Self-Employed to Essential. It would be a case of exporting any information you would like to keep from your self-employed account and saving it to your pc then canceling your account and taking out a new account in QuickBooks on the essentials package.
If you have to open a new QBO account, make sure to click the Buy button to get the discounted price for up to 3 or 6 months. If you purchase from the Trial account, you have to pay the regular rate.