actually, its maybe easier just to edit a file on the device
the file is
/usr/share/ssu/features.d/adaptation-commnity.ini
there is a URL in there for where the port packages are, it should be
https://repo.sailfishos.org/obs/nemo:/testing:/hw:/volla:/yggdrasil:/%(release)/sailfish_%(release)_%(arch)
After editing type
ssu ur
zypper ref
zypper dup
and all should (hopefully) be fine