Connect your phone to Ubuntu desktop
sudo apt-get install phablet-tools
Unlock phone (you need also developer mode on)
sudo system-image-cli --switch ubuntu-touch/rc-proposed/meizu.en
After automatic download and reboot you will have the new channel.
Be warned this channel is not for normal usage.
As Canonical dropped development, one may consider switching to more recent build channel from UBports. Command example below. Run it on Ubuntu host where you have connected your Ubuntu phone.
ubuntu-device-flash --server=http://system-image.ubports.com touch --channel=15.04/rc