It's a subscription so, rather than upgrade, you substitute one subscription with another. When you signed up you had three choices:
Try free for a month - Lucky you. You can abandon this within the month, no cost.
Annual commitment - Cheaper but you are committed for the year.
Monthly commitment - More expensive but you can abandon this at any time.
If you made an annual commitment, I would run your query via MS sales using the "Chat with sales" option on the right hand side of this link. https://www.microsoft.com/en-us/microsoft-365/business/microsoft-365-business-basic?activetab=pivot%3aoverviewtab
In the meanwhile, to overcome your immediate need to use a full fat version of excel, I would make use of the "try free for a month" option for Apps for business available on this page. https://www.microsoft.com/en-us/microsoft-365/business/microsoft-365-apps-for-business?activetab=pivot%3aoverviewtab