Let's take a look at a couple of scenarios and help change the category.
Are you trying to change a category type to an account? If so, you can go to the Chart of Accounts and edit the account you want to change. Here's how:
Click Accounting from the left menu, then select Chart of Accounts.
Find the account you want to edit and click the drop-down beside View register. Then, select Edit.
From the Account Type drop-down, choose the correct category of the account.
Click Save and Close.
However, if you're trying to change the category into a transaction, you'll want to open it and modify the category details. Let me guide you how.