Pidgin 2.12.0 does not have an officially-hosted binary for Ubuntu 16.04, so you would have to build the package from source. Pidgin has documentation for that. Luckily, we can skip building from source altogether; there is a third-party PPA that is already hosting the pre-compiled binary files named Backports.

Installing via third-party PPA

To use it, just follow the instructions as follows:

1. Add the PPA:

   sudo add-apt-repository ppa:jonathonf/backports
   sudo apt-get update
   

2. Install the package from the newly-added PPA:

   sudo apt install pidgin
   

This should add Pigin 2.12.0 to your system.

Installing via building from source

If installing the package from the third-party PPA doesn't work, you can always build from source. As mentioned before, Pidgin has documentation for installing their package from source. In order to make this answer as standalone as possible by not depending on a link that could be moved or die (unlikely), I'll be adding the instructions here, as well.

1. cd to the directory containing the package's source code and type ./configure to configure the package for your system. If you're using csh on an old version of System V, you might need to type sh ./configure instead to prevent csh from trying to execute configure itself.

   Running configure takes awhile. While running, it prints some messages telling which features it is checking for. If it finishes successfully, it will print out of a summary of the build options. This summary will also appear in the help → about dialog.

2. Type make to compile the package. On some systems, you may need to use gmake instead.

3. Optionally, type make check to run any self-tests that come with the package.

4. Type make install to install the programs and any data files and documentation. You must have write access to the prefix you are installing to. See below for more details on the prefix.

5. You can remove the program binaries and object files from the source code directory by typing make clean. To also remove the files that configure created (so you can compile the package for a different kind of computer), type make distclean. There is also a make maintainer-clean target, but that is intended mainly for the package's developers. If you use it, you may have to get all sorts of other programs in order to regenerate files that came with the distribution.

6. If you have not run make distclean, you can use the target make uninstall to remove the files installed by make install.