pip doesn't current contain a flag to allow a user to update all outdated packages in one shot. However, this can be accomplished by piping commands together in a Windows environment:
for /F "delims= " %i in ('pip list --outdated --local') do pip install -U %i
This command takes all packages in the local virtualenv and checks if they are outdated. From that list, it gets the package name and then pipes that to a
pip install -U command. At the end of this process, all local packages should be updated.