There's no built-in command or parameter yet, but you can use:
pip list --outdated --format=freeze | grep -v '^\-e' | cut -d = -f 1 | xargs -n1 pip install -U
In older version of
pip, you can use:
pip freeze --local | grep -v '^\-e' | cut -d = -f 1 | xargs -n1 pip install -U
There are infinite potential variations for this. I'm trying to keep this answer short and simple, but please suggest variations in the comments!