You can cancel pending withdrawal requests by heading to Funds > Transaction History > Pending Withdrawals.
Yes, you can withdraw part or all of your savings through making a withdrawal request in the ‘Transfers’ section of the web, iOS or Android app. There is no limit to how much you can withdraw at one time (subject to a minimum withdrawal of $5 and your account balance must not be less than $5 otherwise you may need to do a full withdrawal).