At the moment the only way to update is to build version 3.2 yourself from source. Very soon version 3.2.1 will be out. At that stage I hope to release a binary .deb file which will be installable on a Pi4 with a simple double click.
The .deb file has been offered to the Raspberry Pi Foundation to include in Raspbian, but they are only interested in a strict .deb source file, which would entail a considerable amount of extra work, as the installation layout required for such is markedly different to the way that Sonic Pi is laid out for other platforms such as Mac and PC. My deb, is of an installation that is placed in the /opt folder, as per the previous sonic-pi 3.0.2 version which was on the Raspbian Stretch release a couple of years ago. Unfortunately this does not run on a Pi4 which requires Raspbian Buster.
In fact the build it yourself option is not too difficult, just a bit time consuming, but can be done within an hour. I will be publishing details of how to do this too, but at the moment there are still tweaks in the build process taking place as the code for version 3.2.1beta alters.