If you’re using the Docker deployment method, you may have an older container which you can restart (check e.g. docker images and/or docker ps -a).
You can also override the commit used to build the Discourse instance by editing app.yml and setting the version to the commit prior to the change, then rebuilding:
params:
version: adb7fa5e2fc51308efc9fc4ee57ecb1c15a85cfa
Discourse will break again if you update after this, which is not ideal given the security update that has been released since (although exploitation potential seems pretty limited for most instances).