You can't change the currency or timezone of an existing ad account once it has been created.
If you need a different currency/timezone, you'll have to create a new ad account with the correct settings. Since you already have 2 accounts in the BM, check Business Settings > Accounts > Ad Accounts > Add > Create a new ad account.
If Meta doesn't give you the option to create another one, you've most likely reached the current ad account creation limit for that BM. Having a 6-year-old BM doesn't automatically mean you can create unlimited accounts.
In that case I'd keep the existing accounts as they are and check the Business Info section for your ad account creation limit. Don't try to change it through random workarounds, especially if the BM has history and is healthy.