How to upgrade to master branch of LinuxCNC for rpi4 from LinuxCNC 2.9.3

For an x86 (Standard PC) system you can change your repository settings to point to the buildbot: buildbot2.highlab.com
But as you can tell if you look there, we don't have any arm64 machines building images there.

So, that just leaves building from source. Which is actually fairly straightforward. Also, if you build from source you can have both versions co-existing on the same machine, if you want.

linuxcnc.org/docs/devel/html/code/building-linuxcnc.html

If you want to have the very latest version then don't try the tarball route, go straight to using Git. That way each update is just a case of "git pull", followed by "make".

Compiling "run in place" as described there is quickest and most adaptable, but it does mean that you need to perform the incantation:
(Yes that really is dot space dot slash)
To start Linuxcnc. But you can make a desktop shortcut to do this.

If you build and install a .deb then it will look exactly like a standard installation, with the downside that every update will take about an hour to buid and install, rather than a few seconds.

You will need to install git, and a whole lot of dependencies. Don't be surprised by this.