Here is a one liner (choose the line depending on your operating system, Linux, MacOS or Windows):

curl -fLO https://github.com/rordenlab/dcm2niix/re...
curl -fLO https://github.com/rordenlab/dcm2niix/re...
curl -fLO https://github.com/rordenlab/dcm2niix/re...

extract the archive and make sure that this new downloaded executable is in your path. On Unix operating systems you can run "dcm2niix -u" to check if your version is up to date.