This can be done in the device/group settings, in the channel unit settings area. All objects beneath the changed one inherit that setting by default. Note that you may have to recreate the sensors in order for this to be updated to the new units.
- Click on the device so that you're seeing all of its sensors
- Click on the settings tab
- Scroll down until you see the section Channel Unit Configuration
- Untick the checkbox beneath it where it says Inherit From ...
- Configure the units in the first section (where it says Bytes (Bandwidth))
- Save