How to change from Ubuntu touch stable channel to rc-proposed

Install the Terminal app from the store (or if you have enabled developer mode, you can use the phablet-shell tool over USB connection). If using Terminal app on the phone, you may need to disable screen locking Battery Settings first, as if the screen locks, the Terminal app (and its child processes) will be stopped. You can then change the setting back after switching channels.

Then, on the phone, run sudo system-image-cli -vvvv --switch $newchannel where $newchannel is the new channel you switch to switch to. For different devices, this channel may be different. For example, on the BQ Aquaris E4.5 and E5 Ubuntu Edition phones, the channel should be ubuntu-touch/rc-proposed/bq-aquaris.en to get rc-proposed. You can use system-image-cli -i to show current information, and generally just replace the stable portion of the existing channel name with rc-proposed to get the right channel. You can also perform the same action in reverse to switch back.

It is also possible to follow the instructions here, it appears to only change the system partition: https://developer.ubuntu.com/en/phone/devices/installing-ubuntu-for-devices/

I think these are the important steps (on an Ubuntu desktop):

1. Add the Ubuntu SDK Release PPA:

   $ sudo add-apt-repository ppa:ubuntu-sdk-team/ppa

2. Update your system to use the latest packages:

   $ sudo apt-get update

3. Install the ubuntu-device-flash package

   $ sudo apt-get install ubuntu-device-flash

4. switch the channel, e.g. for the bq M10:

   ubuntu-device-flash touch --channel=ubuntu-touch/rc/bq-aquaris-pd.en

Your tablet needs to be running, with developer mode enabled (and connected via USB to the desktop). You do not need to boot into recovery mode. If it doesn't work keep trying, it doesn't always work on the first attempt.