I'm here to help you change the currency on the QuickBooks Self-Employed (QBSE) mobile app, userinfo44.
You need to open your account to your specific country. The multi-currency feature is available in QuickBooks Online and Desktop, not on QBSE. Hence, you're unable to change the default currency on your mobile app.
If you're located in the United Kingdom, then you have the correct app. If you're in the United States, you can download the app through this link: https://quickbooks.intuit.com/accounting/mobile/.
Let me know if you need anything else aside from converting the currency displayed on your mobile app by commenting below. I'm always around to help you.