Ubuntu Touch using apt-get

From Installing Ubuntu on devices:

Enabling read-write mode

By default the system is read-only. You can switch to read-write mode, although this disables Ubuntu system upgrades. The main purpose for this is developing the Ubuntu system directly. This is not required for developing apps or using the system normally. Recovering from read-write mode is possible but requires reinstalling the system from scratch.

Warning: Switching a device to read-write mode (and/or recovering from it) is an advanced feature and may result in complete data loss.

Warning: Switching a device to read-write mode disables automatic over-the-air updates.

Install on your host system the phablet-tools package:

sudo apt-get install phablet-tools

And type the following command:

phablet-config writable-image

The system will reboot in read-write mode, but even if you'll be able to use apt-get or set up a PPA, only packages built for armhf will be installable.