Yes, at any time. If there's money set aside for your rent or mortgage, you'll see a Withdraw button on your rent and mortgage screen. It goes back to your own bank account.
If there's ever more in there than your payment needs, say because you added some manually as well, the extra just stays put. We only ever send the amount you told us, and we won't use the rest for anything else. Leave it there for next time, or withdraw it.
One thing worth knowing: if you're a credit card customer or have auto top-up enabled, we'll still collect the full amount from your bank next time, so anything extra sits there rather than going towards your next payment. Withdraw it whenever you like.