I have the same problem and I solved it and the explanation is as follow:
If the Seaborn package is not installed in anaconda, you will not be able to update it, namely, if in the Terminal we type: conda update seaborn
it will fail showing: "PackageNotFoundError: Package not found: 'seaborn' Package 'seaborn' is not installed in /Users/yifan/anaconda"
Thus we need to install seaborn in anaconda first by typing in Terminal: conda install -c https://conda.anaconda.org/anaconda seaborn
Then the seaborn will be fetched and installed in the environment of anaconda, namely in my case, /Users/yifan/anaconda
Once this installation is done, we will be able to import seaborn in python.
Side note, to check and list all discoverable environments where python is installed in anaconda, type in Terminal: conda info --envs