I don't dual boot - I've only ever flashed my device using
ubuntu-device-flash, which takes a --channel argument. So something like this
will flash you to utopic:
$ ubuntu-device-flash --channel ubuntu-touch/utopic
Note the extended channel name containing a slash. Channels were renamed a
while ago. ubuntu-device-flash has a --list-channels option you can use to
list all available channels.
If you have shell access to your device, once you're booted into Ubuntu, you
can use the system-image-cli command to switch channels too. Read the
system-image-cli manpage for more details, however it's pretty easy.
To see all available channels:
$ system-image-cli --list-channels
To switch channels, e.g.:
$ system-image-cli --switch ubuntu-touch/utopic
However, note that --switch only works with system-image 2.3.1 which isn't
available in Trusty. No matter; --switch is just an alias for
$ system-image-cli -b 0 -c ubuntu-touch/utopic
Cheers,
-Barry