Based on this answer from Ask Ubuntu, there is a command line option to install the previous version of a package, but it doesn't look like Elementary's repositories still have it available:
sudo apt-get install --reinstall switchboard-plug-power=2.3.4+r609~ubuntu5.0.1
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '2.3.4+r609~ubuntu5.0.1' for 'switchboard-plug-power' was not found
So, Elementary doesn't seem to keep old versions (at all) in their repository manifest. However, I was was able to find them available for direct download on Elementary's server. So for example if you had an AMD64 system, you would download that .deb file and install it manually.
After downloading the file, use
dpkg to manually install the older version. For example, if I did that with my Intel 64-bit computer, I'd do:
sudo dpkg -i switchboard-plug-power_2.3.4+r609~ubuntu5.0.1_amd64.deb
It would then proceed to downgrade the package:
dpkg: warning: downgrading switchboard-plug-power from 2.3.5+r634~ubuntu5.0.1 to 2.3.4+r609~ubuntu5.0.1
(Reading database ... 306254 files and directories currently installed.)
Preparing to unpack switchboard-plug-power_2.3.4+r609~ubuntu5.0.1_amd64.deb ...
Unpacking switchboard-plug-power (2.3.4+r609~ubuntu5.0.1) over (2.3.5+r634~ubuntu5.0.1) ...
Setting up switchboard-plug-power (2.3.4+r609~ubuntu5.0.1) ...
Processing triggers for dbus (1.12.2-1ubuntu1) ...
If you want to prevent future automatic upgrades, you can run
apt-mark hold switchboard-plug-power.