Log into your account. Tap + right next to your balance - you'll see the 'Deposit' button in purple. Select a payment method, add the desired amount and tap 'Deposit'.
If you're using your computer, click on the 'Deposit' icon in the top right corner - you'll be taken directly to the deposit page.
The minimum deposit amount is 10 GBP / 10 EUR / 10 CAD.