You can withdraw rewards on the Rewards page of the Token Dashboard. You must withdraw per signing group because of the gas costs and signing group selection process design.
It is not possible to withdraw all available rewards in one transaction because we either risk failing on out of gas or this becomes expensive as we'll have to provide a lot of input parameters.
You may notice that you can’t withdraw all of your rewards even though you have rewards in your balance. Your rewards balance includes both available and accumulating rewards. You can be continuously earning a reward in an active signing group but may not be able to withdraw it until the signing group expires. Those are marked in the table as "Accumulating."