cd ~/Downloads/cmake-3.12.0-rc3/   # or wherever you downloaded cmake
./bootstrap --prefix=$HOME/cmake-install make make install export PATH=$HOME/cmake-install/bin:$PATH export CMAKE_PREFIX_PATH=$HOME/cmake-install:\$CMAKE_PREFIX_PATH

The two export lines make sure that your updated cmake is used instead of the system cmake. You have to make sure that those lines are executed whenever you want to use the updated cmake. It's probably a good idea to put them into your .bashrc.