I have the same issue with ubuntu server minimal for Raspberry Pi 2 (downloaded from https://ubuntu-pi-flavour-maker.org/download/).
To downgrade:
From command line (terminal) you can run
apt-cache policy chromium-browser
Now you can see the versions that are available.
chromium-browser: Installed: 45.0.2454.101-0ubuntu1.1201 Candidate: 49.0.2623.108-0ubuntu0.15.10.1.1223 Version table: 49.0.2623.108-0ubuntu0.15.10.1.1223 0 ... etc etc
At least for me, there are 2 versions available, where the oldest works.
I had to remove the newest with
sudo apt-get remove chromium-browser chromium-codecs-ffmpeg-extra chromium-browser-l10n
and then install the older one with
sudo apt-get install chromium-browser=45.0.2454.101-0ubuntu1.1201 chromium-codecs-ffmpeg-extra=45.0.2454.101-0ubuntu1.1201 chromium-browser-l10n=45.0.2454.101-0ubuntu1.1201
(Edit to add Aworanās instructions to block chromium updates)
Block updates with
sudo apt-mark hold chromium-codecs-ffmpeg-extra chromium-browser chromium-browser-l10n