To top up your fiat account go to Accounts and press Top Up button in the top right corner. Pay attention that only SEPA transfers in EUR are available now.
You will see your IBAN details.
After that, you can visit your bank or make a transfer using your banking app (if it allows to do so). Check with your bank if they support SEPA payments.
Make sure you enter 'Transfer own funds' as the purpose of payment. Otherwise, your transaction will be declined.
After successful top-up, you will be able to check your balance in the Accounts section.