To disconnect your current reader from your iOS device, first click the drop down menu within the app. Then, click 'My Devices' 

disconnecting a reader menu

Once your device screen has appeared, click 'change device'. An alert should appear with the following text: "are you sure you want to change device as this will disconnect the current one". Click 'OK'

disconnecting a reader 2

This will take you to your settings. Click on the arrow by 'Bluetooth' to get to the Bluetooth menu. 

disconnecting a reader 3

A list of your devices will appear. Click on the info icon.  You will then be prompted to either 'Disconnect' or 'Forget This Device'. Click 'Forget This Device'.