Let me provide some overview of why you're unable to change your home currency, Janna.

Home currency is the currency your business uses for its transactions based on where your business is located. Once you've set them up, you're unable to change to a different currency. 

You'll want to purge your data so you can start fresh. Please know, that doing so deletes the data in your QuickBooks Online (QBO) account. To delete your company data, follow the steps outlined below:
 

1. Sign in to your QuickBooks Online account.
2. Important: You'll need to sign in as an admin. If you're already signed in, make sure it's the company you want to purge.
3. Once you're signed in, open this link. Alternatively, you can go to the following web address:
   • https://app.qbo.intuit.com/app/purgecompany
4. To confirm you want to delete your company data, enter Yes, then select OK.
5. Select your industry from the ▼ dropdown.
6. Select Create accounts based on my industry. This helps QuickBooks set up your chart of accounts after your data is wiped. Don't select Create an empty chart of accounts unless you've checked with your accountant first.
7. Select Wipe data.

The above steps are only applicable if your books are less than 60 days old. After purging your data, ensure to select the correct currency before enabling multicurrency. 

However, if your books are more than 60 days old, check out this article for more details: Delete Your Data And Start Over In QuickBooks Online.

Furthermore, learn how QuickBooks uses exchange rates to get the home currency value of your foreign currency transactions. I've added this article for reference: Learn About Exchange Rates in QuickBooks Online.

Reach out to us if you need furthermore assistance with currencies. I'd be glad to help.