How do I delete sub-categories that I've added?

You can delete a sub-category that you'd added, but only if you don't have any transactions associated with it.

One quick way to verify is to look at your Profit and Loss report (https://app.sellerledger.com/reports) - if the totals for that sub-category show $0.00, then you should be

If you go re-categorize those as another sub-category, then, when you go to https://app.sellerledger.com/settings/categories and click the pencil icon next to the subcategory you wish to remove, there will be a "Delete" button to the right, like the screen below:

If that "Delete" button is not visible, it means there are still some transactions in your account that use that sub-category. Go find those transactions, re-categorize to another category/sub-category, and then you should be able to delete this unused on.