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Originally created by @intrigeri on #6591 (Redmine)

A GUI to select another upgrade channel (e.g. alpha or beta) would make it much easier:

1. for us to document how to upgrade to a Tails release candidate;
2. for users to follow this documentation.

Implementation-wise:

• If implementing this in Perl: Tails::RunningSystem in our perl5lib allows to trivially get the currently configured channel. Adding the needed backend code there to change this setting and write /etc/os-release should be pretty easy. Adding a frontend on top should also be easy.
• If implementing this in another language (Python, shell): either do the /etc/os-release parsing and writing yourself; or preferably add a short Perl wrapper executable around our perl5lib, and build a GUI on top using your preferred programming language; this way, the channel reading and writing is kept in a single place, instead of being duplicated in various programming languages; intrigeri may be willing to write the needed Perl bits if it helps.