1. Work out which situation you're actually in. "Delete a company" usually means one of two things: wipe a test or newly-started company and begin again, or shut down one company while a different one keeps running. The steps aren't the same.
2. To start over with a new company, check the window first. Purge Company only works within 60 days of the company's creation date, and only if you haven't processed payroll in it yet. If either of those has already happened, this option is gone.
3. Open the Purge Company tool. While logged into the specific company, go to its URL and add /app/purgecompany to the end, or search "purge" from QuickBooks' own help search inside that company to get the direct link. It isn't tucked into a regular settings menu.
4. Confirm and purge. QuickBooks asks you to type a short confirmation phrase before it runs. Once confirmed, every transaction, list entry, and customization in that company gets wiped, but the company shell itself, its name and subscription, stays in place for you to rebuild from zero.
5. To close one company while others keep running, go to Billing & Subscription instead. Open the specific company you're closing, click the gear icon, then Account and Settings, then Billing & Subscription, and cancel that company's subscription. Each company under your Intuit account bills separately, so this doesn't touch anything else.
6. Check your company switcher afterward. A canceled company drops out of your active list but, like any cancellation, stays viewable read-only for a retention period before it's actually purged for good.