You can withdraw bitcoin from Bitstack to any other Bitcoin compatible wallet at any time. To do so:
Tap on the ⬆️⬇️ button in the app
Enter the amount of bitcoin you want to withdraw
Scan a QR code or tap Send to enter the recipient's BTC address
Tap Continue to verify your withdrawal details
You need to withdraw at least 0.00100000 BTC or 100,000 sats (satoshis) to initiate a transaction.