sudo apt-get install pkg_name=version

Or, you may download the version you would like to install and keep it in a folder, say abc.deb in Downloads. Open terminal, move to the folder using cd command and install the previous version usingdpkg:

sudo dpkg -i abc.deb

OR you can use ppa:

sudo apt-get istall ppa-purge

Then you can remove the ppa using command

sudo ppa-purge ppa:user/ppa-name

this will autometically downgrade the software to its original version which shipped with ubuntu.