Welcome to the Community family! Mistakes sometimes happen, and I know how important it's to resolve them. QuickBooks Online not only automatically sets up different account types for you to categorize transactions but also gives the flexibility to add new accounts or edit their configuration afterward if needed. I'll be happy to share more on this.
From what you're describing, it sounds like a category that isn't set up as you'd like was used across your transactions. If that's the case, you can simply edit its configuration in your chart of accounts and this will automatically impact every corresponding transaction in your books. Here's how to proceed.
Go to Accounting from the left menu then Chart of Accounts.
Locate the account/category you'd like to edit.
Select the drop down arrow next to Account history or Run report (depending on the account).
Select Edit.
Make all desired changes and click Save and Close.
This route is quicker than creating a new account/category and editing each transaction manually, but feel free to do so if you'd prefer this way. Here are some articles to assist you with this.