conda install -c conda-forge iris
If you wish to run any of the code examples (see http://scitools.org.uk/iris/docs/latest/examples/index.html) you will also need the Iris sample data. This can also be installed using conda:
conda install -c conda-forge iris-sample-data
Further documentation on using conda and the features it provides can be found at http://conda.pydata.org/docs/intro.html.