After placing an order on the exchange, those funds you are trading are regarded as "reserved funds" -- they are reserved and cannot be exchanged or transferred elsewhere until the order is either filled or cancelled (for limit or stop orders).

After placing a Limit or Stop order, you can still cancel the order by going to the tab 'Orders' and selecting 'Cancel'. This will also free up those reserved funds.

On the mobile app, navigate to "Order Management" and you can see any open orders you have, with the option to cancel them there as well.