If you use anaconda to manage your Python environment, run this command in your terminal:
$ conda install -c conda-forge specutils
Otherwise, the recommended method is using pip:
$ pip install specutils
These are the preferred methods to install Specutils, as they will always install the most recent stable release.
The sources for Specutils can be downloaded from the Github repo.
You can either clone the public repository:
$ git clone git://github.com/astropy/specutils
Or download the tarball:
$ curl -OL https://github.com/astropy/specutils/tarball/main
Once you have a copy of the source, you can install it with:
$ python setup.py install